I believe there is room for a private social network that is peer-to-peer, encrypted, with no central servers and an open spec. It would make my world a better place, but I don't think it can be monetized. (Maybe donations? non-profit?) Also, privacy breaks down when you run a client on Windows 8 or above. :-( I think a lot of tough computer science challenges are inherit in this model. And yes, there is the network effect dilemma, but I don't require mainstream adoption to use it.