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.