Of course, making these sorts of predictions is a parlor game: Almost any prediction you make rests on analogies no one can be sure are relevant, or impressions of the future no one can test. Nevertheless, curiosity impels an effort.
Facebook may be forever for a few reasons, only one of which is the sheer number of people -- 140 million Americans -- on the site. That's a much larger percentage of Internet users than any other site has attained, as you can see in the chart above. And it's not simply the size of Facebook that matters, but who Facebook has reached. It's no longer the early adopters or even the second- or third-wave joiners. Facebook is now an Internet home to some very late adopters -- such the people who have not and will not leave Yahoo!, AOL, and Hotmail for Google's web services, and many of the Internet's 65-and-older population, who now make up 6 percent of Facebookers. These people matter to the site's staying power more than the early adopters for a simple reason: If a new site comes along, sure, I'll check it out. But if I want to be in a social network with my mom, my aunts and uncles, some professors from college, Facebook is going to continue to be the place where I can find them.
The other reason is the site's archive of our lives since we joined. The permanence of these memories on the site may unnerve some people (as well it should), but it is also a feature: The collection of those memories is hard to give up. When I've thought about canceling my Facebook account for one reason or another, the thing that's stopped me is the access to years of photos of me and my friends and family, the notes and messages we've written, and the connections -- however thin -- to people I've long since fallen out of touch with on every other mode of communication. Were I to join a new site, I would never have the nerve to "friend" (or whatever the equivalent of that is in the future) the people from high school, college, and long-ago travels who currently appear among my Facebook friends.
There are still reasons Facebook could falter: If it fails to adapt to changes in how we live over the next several decades or if its recklessness with users' privacy becomes intolerable. But both of these seem unlikely causes of Facebook's demise, for different reasons -- the former because Facebook has shown itself to be capable of adapting, as in its quick assimilation of smartphones into its reach, and the latter because it seems that people's tolerance for Facebook's recklessness is quite high, at least when weighed against what they like about the site and what they feel like they would lose by leaving. Or perhaps kids of the future just won't want to join a site their parents have already conquered and claimed as their own. Who knows. But that doesn't seem to be deterring them yet.