I spent a lot of time thinking about this stuff at one time; when I was sorta-kinda playing with the idea of trying to do something that had some potential as a "facebook killer." But that was never my real aim anyway, and I wound up refocusing a lot of my energy in other directions now. But I still find the idea of thinking about "what will replace Facebook" interesting. If you ever want to bat some ideas around, feel free to shoot me an email.