Facebook, MySpace, LastFM... would work, if everyone you knew was a member. Problem is that is never going to happen. The thing is that social networks are walled gardens, built so that your data once in, stays in.
I want to retain ownership of my data! If I want sign up to a new better service next week I dont want to have to resubmit everything again, similarly the week after. Why can't my social networks be social?
Comments? Tweet me @mealybar, smoke signals, or homing pigeon, or something :)