The thing with moving to another social network and leaving the old one behind is that you won't find it to be too social without your existing network of friends/family moving over as well. It's a bit of a Catch-22 situation, really.
Of course, it's possible to use your existing networks to invite people to a new network, but in reality, unless people see value in the new vs the old, they will at best politely decline. In Facebook's case, you'll need people's concerns with privacy to outweigh the inconvenience of switching and building a new network.
I've been invited to about 5-10 new 'networks' from people on Facebook or LinkedIn over the last couple of years. I haven't moved partly because some of them look kind of dodgy to be honest but most because I'd simply have to start over again. That's the 'lock-in' that online social/business network tools have.
People need to be convinced that the inconvenience of switching outweighs the existing concerns.
Facebook is the first thing, the ONLY thing, that's gotten hundreds of people I know (I mean, really know) to all be in the same "place" at once.
I abandoned Friendster and MySpace long before Facebook showed up. Neither of them ever truly reached the critical mass that Facebook did IMHO. If you remember what Friendster and MySpace were like in earlier days, you kind of just friended people and sat there. The engagement on Facebook is MUCH higher.