Now I get it!
I think somewhere along the way I had misunderstood the whole social networking software movement. I had been thinking that all of the applications were trying to run one service that everyone would sign up for, but the recent proliferation of services makes it clearer what's actually going to happen.
In the future, everyone will have his or her own social software application, and then try sending out invitations to their friends to have them join it. Everyone's worried about the services being centralized, but with five billion of them sprouting up, we'll be completely decentralized. It'll be just like our email address books!