The problem with generic social networks like Facebook and Twitter is that people do not have generic identities. Our identities are many faceted and complex which is part of what makes people so wonderfully and intriguingly unique. We play different roles in different contexts, and when those contexts merge our worlds collide and things get ugly, or at least annoyingly complex to juggle. Consider when your parents join Facebook or your boss asks to be your Facebook friend.
In the offline world there is no such thing as a generic social network. All our social networks are niche social networks where we get to nurture parts of our complex identities and fulfill some of our many social needs.
This is why I believe that our current concern with generic online social networks like Facebook and Twitter will eventually pass as we all re-discover our various identities in online niche social networks. People may still want to have a presence in the generic social networks, but the majority of user-created content, self expression, and activity will be distributed across our various niche social networks.