At this point, just about anyone joining Google+ has already joined a few social networks: Friendster, MySpace, Facebook and Twitter, but also a few others probably, too: Instagram, LinkedIn, Gather, Ping, OKCupid, Rdio.
Myself, I've built a bunch of different networks in different ways. On Friendster, I assiduously cultivated a small group of people who never used the service again. On Facebook, I just let whatever happen and that lack of care created a crappy network (I've since deactivated). Twitter I built into my personal newswire, along with random deep dives into some fields like energy that I've spent a lot of time researching. Instagram is just for real-life friends. With Rdio, I look closely at a person's music taste before connecting.
My point is: I had already cobbled together my own personal Circles by using different social networks. So, now, what I'm doing with Google+ draws on the lessons I learned *from* all of them. Friends is Instagram. Acquaintances is Twitter. Family is still family.
Google+ has provided the technology and fresh start, but I'm bringing improved technique to my social networking. I've learned and changed and want different things from my social networks than I did two or ten years ago.
Maybe some people don't and they'll be happy sticking with Facebook or Twitter forever. But for those that do, Google+ is an opportunity to consolidate their social networking selves into one place.
In a sense, this is a counterargument to Yishan Wong's post on Quora, "How Google+ Shows That Google Still Doesn't Understand Social." His excellent statement of Facebook's position points out that no one uses Facebook lists.
When the friends list functionality was originally developed at Facebook, one of the critical realizations was that regular users simply did not and could not comprehend how the feature worked. Something like 1-5% of users were willing to use this feature even when it was prominently displayed to them in numerous UI formulations, each one laboriously A/B tested to see if there was a way to get more users to be willing to sort their friends into lists. Instead, the friend list functionality simply confused most normal users and introduced confusion and complexity to the friending process, so it was appropriately relegated to the status of being an "advanced" feature for privacy-conscious power users.
Yet I would venture that the vast majority of Facebook's users are also on other social networks that they compose differently. It's not that only nerds sort their friends; it's that only nerds sort their friends on Facebook.
Wong also underestimates the changes that many people I know have experienced with regard to social networking. It's not surprising to me that a few years ago people weren't interested in controlling their online reputations or privacy. Now, using my admittedly small circle of good friends and family as data, people are wising up.