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In the world of Harry Potter, drinking the blood of a unicorn will keep anyone alive, “even if you are an inch from death.” But survival comes at a terrible price: “You will have but a half-life, a cursed life, from the moment the blood touches your lips,” the centaur Firenze explains to Harry.

Facebook is unicorn blood, and it has touched the metaphorical lips of billions of relationships. Of late, the social network has been attempting to absorb every facet of existence formerly untouched by its grasping fingers: dating, commerce, etc. But its central feature remains what it has always been—a digital Rolodex of everyone you know and everything those people have ever shared on their page. Your “friends.” Some of these people are actually your friends, of course. And some of them are old Little League teammates, or people you took one class with in college, or fourth cousins you’ve never met but found on Ancestry.com.

These relationships are the ones that suffer most from Facebook’s shimmering unicorn-blood curse. They live an extended half-life far beyond their natural life span, hobbling on, an inch from death, in the form of likes and invitations to multilevel-marketing groups and news-feed photos of children you’ve never met and don’t care about. Facebook itself has built in ways of reminding you of the connections it has cataloged: For certain friendships chosen by the algorithm, the site sends notifications and shows you a video slideshow on your "friendiversary"—the date one of you clicked Confirm on a friend request.

The social network is 15 years old this Monday, and in taking stock of the effects of its decade and a half of existence on people’s social lives, this is what stands out the most: Facebook is where friendships go to never quite die.

The site has created an entirely new category of relationship, one that simply couldn't have existed for most of human history—the vestigial friendship. It’s the one you’ve evolved out of, the one that  would normally have faded out of your life, but which, thanks to Facebook, is instead still hanging around. Having access to this diffuse network of people you once knew can be pleasant—a curio cabinet of memories—or annoying; if those good memories get spoiled by an old friend’s new posts; or helpful, if you need to poll a large group for information. But it is, above all, new and unusual.

It’s reasonable to assume that those with older Facebook accounts are likely to have more of these lingering vestigial friendships, or, as researchers often call them, “weak ties.” They’ve had more time to “friend” people and then fall out of touch. But even those who’ve joined the site more recently may experience this phenomenon. For instance, Rebecca Adams, a sociologist with the gerontology program at the University of North Carolina at Greensboro, just got a personal Facebook account for the first time last year. (She had a professional Facebook account already.) And she optimized her account so she could find some of these estranged connections: “I used my high-school picture when I got on Facebook,” she says. “I already knew which people would recognize me now, but what I was interested in was finding the people who knew me when I was young.” There’s a sort of shared understanding that connecting with people you’ve lost touch with is, at least in part, what Facebook is for.

Fifteen years into the Facebook era, it’s well established that people aren’t actually friends with the hundreds or thousands of Facebook friends they may have. They couldn’t be if they tried—research has found that there seems to be a limit to the number of social connections a human brain can manage. Robin Dunbar, an anthropologist at the University of Oxford, is the most famous proponent of this theory, and his estimate of 150—known as “Dunbar’s number”—is often cited as the (approximate) number of casual friends a person can keep track of. There are different Dunbar numbers for different levels of closeness—concentric circles, if you will. The smallest circle, of five friends, consists of someone’s most intimate friendships. One can keep track of 15 close friends, and 50 pretty close friends. Expanding out from the 150 casual friends, this research suggests that the brain can handle 500 acquaintances, and 1,500 is the absolute limit—“the number of faces we can put names to,” Dunbar writes.

Pew data from 2014 (the most recent available) found that the average Facebook user has 338 friends, and half of users have more than 200. (Younger people tend to have more than older people.) And it’s not uncommon, when browsing the site, to run into a person who is pushing 2,000 friends. Many people’s Facebook networks extend far into Dunbar’s acquaintance circle, at least.

Some observers over the years have worried that large networks on Facebook would lead to people spreading their limited cognitive resources too thinly, sowing their wild likes over too many acres of shallow acquaintances, rather than diligently tilling the soil of their closest relationships. It’s still unclear whether people actually use the site that way, partly because Facebook is a black box that rarely shares such data. (The company declined to provide any numbers for this article.) But it may well be that people are largely ignoring the drumbeat of updates from their weakest ties, and focusing most of their energy on a select few people. Even though “you’re finding out about people you would not have otherwise kept up with, I think people are still disproportionately communicating with close ties,” says Robert Kraut, a professor of human-computer interaction at Carnegie Mellon.

But users still have an “ambient awareness” of Facebook friends they aren’t communicating with. As one study defines it, this is “awareness of social others, arising from the frequent reception of fragmented personal information, such as status updates and various digital footprints, while browsing social media.” Basically, you probably know if any of your old high-school friends are pregnant right now, whether or not you’ve spoken with or even actively checked up on a given person, as long as you’re both on Facebook. You’re no longer sharing life experiences or creating memories with these weak ties, but as you live your separate lives, you’re forever in each other’s periphery.

Kraut found, in a study he did in conjunction with a researcher from Facebook, that just passively consuming someone’s posts is linked to feeling closer to that person. But the effect on closeness that they found was stronger for relationships where Facebook was the only means of communication. And it stands to reason that just reading acquaintances’ Facebook posts makes you feel closer to them than not hearing from them at all.

“I think of that as a reservoir,” Kraut says of the network of weak ties Facebook keeps alive. “Most of them you’re not going to regain intimacy with, even if at one time you were intimate. But they are available in times of need.”

Nicole Ellison, who studies social media and online communication at the University of Michigan, has found in her research that when people call on their Facebook network for help, they tend to get it, whether they’re looking for information, emotional support during a tough time, or even someone to help them with an in-person favor, like lending them a book or driving them to the airport.

And, she says, “it may very well be the case that my close friends are not the ones who can give me the information I need, that I really do need to turn to these weaker ties.” Research has also shown that people’s weak ties are often less similar to them than their closer friends, meaning that those ties may often be the way someone is exposed to new perspectives. This could manifest in the phenomenon of the high-school classmate who won’t stop posting political rants you disagree with, or it could lead to a random old acquaintance having just the information you need at just the time you need it.

Having many Facebook friends, then, is kind of like having a big old encyclopedia (in the days before the internet). Most of the time it’s just gathering dust on a shelf, taking up space, but you keep it around anyway, because one day you might need it. Of course, some people do unfriend their weak ties, if they find them irritating or just want to keep their network more intimate. But Facebook lowers the cost of keeping these people at the edges of your life—you have to do something to get rid of them—and so you may feel that you might as well keep them around.

Researchers tend to think of friendship as the most flexible sort of relationship—friends can drift in and out of your life without necessarily losing the title of friend in a way that romantic and family relationships can’t quite. And relationships can move fluidly between Dunbar’s circles of intimacy—your five closest friends six months ago may not be the same as they are today. So some of these weak Facebook connections may just be dormant, and easily reactivated if you were to, say, visit the person’s city and meet up.

Still, many of these friendships that have withered to the point where they exist only on Facebook would not be able to survive without it. “Friendships are environment-specific, and Facebook is its own environment,” says Michael Harris, the author of The End of Absence: Reclaiming What We’ve Lost in a World of Constant Connection. “It is a petri dish in which anything that has the possibility of growing will grow, in terms of social connection. If you took those friendships offline, they would wither and die.” If Facebook’s unicorn blood is a relationship’s only source of nourishment, its life is a fragile one.

People’s close friendships tend to exist across platforms. Research on “media multiplexity theory,” as I’ve previously reported, “suggests that the more platforms on which friends communicate—texting and emailing, sending each other funny Snapchats and links on Facebook, and seeing each other in person—the stronger their friendship is.” In an interesting study from 2012 that surveyed college students on what they thought the “rules” for friendship on Facebook were, several of the rules they came up with had to do with not limiting the relationship to Facebook alone. For example: “Don’t add someone as a Facebook friend unless you meet them offline first,” “I should communicate with this person outside of Facebook,” and “I should wish this person happy birthday in some way other than Facebook.”

If one were to try to take a friendship that has been Facebook-only for several years and move it off Facebook—to texting, say—it would be … weird. And possibly even creepy.

“A channel switch is always laden with some kind of symbolic import,” Ellison says, “especially if you’re moving from a less intimate to a more intimate channel.” Of course, the complex and unspoken etiquette rules of digital communication can be challenging for the best of us to grok, especially given that they tend to differ from generation to generation. A pure and earnest attempt to connect can easily become uncomfortable if filtered through the “wrong” medium. However, such a violation probably wouldn’t be a big deal if it came from a close friend. Why did my best friend, Susan, DM me about her birthday party on Twitter instead of just emailing an invite? Ah, well, you know Susan, she’s so random. Gotta love her, though. But if Kenny from kindergarten, who is your Facebook friend but nothing more, suddenly slid into your DMs, well, that might feel alarming.

“When you have a close relationship with someone, nothing makes as much of a difference as it does in a casual relationship,” Adams says. What’s a social-media norm violation here and there among friends? But if Facebook is the only place your friendship lives, what you do on Facebook matters more. Adams believes that Facebook has likely not had a huge effect on people’s relationships with their closest friends, because they tend to communicate in many different ways. “I think it has more impact on that outer circle [of friends],” she says.

This is the bargain of drinking Facebook’s unicorn blood. It will give you powers heretofore unwielded by man—a council of everyone you’ve ever met who can be summoned to advise on matters of great and small import with the click of a button. But in return, you must watch the hollow shells of those relationships limping along every time you log on.

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