The problem is most people are computer illiterate. Hell, illiterate in general.
So there would have to be some host that has an easy to use personal social network object thingy and well, there you have it: new platform.
Until there’s some kind of open source standard that anyone can obtain, setup, host, use and maintain all on their own without learning more than the most minuscule amount of new knowledge, the platform will be where they flock. People just don’t want to learn anything.