I think part of the problem is the network effect. Social networks want to maximize the number of users so that anyone can connect to you on Facebook, Twitter, etc. The lowest barrier to entry is free, and that usually means ad supported (and personalized ads for the most revenue).

Maybe there's some space for a freemium model (IIRC one of the questions asked during the Facebook hearing was whether they could add a paid ad-free option) but so far that hasn't happened.