The problem is, after an extended use maybe we feel sick of it all. Facebook was new and fresh when it arrived, but now it's beginning to feel like an overstayed welcome. They have accomplished the goal of making life seem, in a way, commoditized, and that makes me feel like I never want to use such a service again.
I don't think social networks are going to be entirely displaced, maybe just the ones that attempt to mirror your personal life. I think it's a kind of impedance mismatch issue--a kind of uncanny valley vibe that they give off. After doing it so long, it just feels so artificial and flaky.
I know everyone doesn't have such negative thoughts about Facebook, but I also know I'm not the only one who feels this way right now. I can't see what they or anyone could do to fix this problem, but I know a solution isn't out of the realm of possibility either.
I predict future successful social networks will be somewhat like HN or reddit--semi-anonymous, with no real connection to real life. If they can surround you with peers clustered around your interest profile, perhaps we could grow organic communities around professions and hobbies that dynamically or elastically determine membership and open channels of communication (as opposed to the current process of self-selection and direct messaging). I don't know, I'm rambling now.