I know that I may be late to the game on this one, but I recently finished reading Smart Mobs by Howard Rheingold. Prior to reading the book, I had been thinking a lot about the potential for peer-to-peer (P2P) social networks to become the successful model of the "online" social network. Reading the book helped solidify my thoughts in this area - and expanded the scope of my thinking to other related areas and possibilities.
While there is currently a lot of talk about online web-based social networks, mobile-web-based social networks will soon become the major form of online social networks. Social networks will have both fixed and mobile interfaces, of course, and the mobile interface will become the primary one.
As I have said before, mobile or fixed, for online social networks to be truly successful, they have to be open. That is, no service provider should define or limit the scope of my social network. As it has been for thousands of years, I will continue to define my social networks - people I am socially connected to should not be required to join one - or fifteen - "social networking services". There is a role for service providers to provide interfaces and services, but my relationships should be defined independently of the service provider.