Have you played any number of "social games" on Facebook? I have, mainly at the behest of my girlfriend. Normally I play FPS, RTS, RPG type games, starting from the late 1980's. And I've played a fair few MUDs since the telnet days.
Facebook games are OK-ish for a while, but the "social" is only skin deep, and every game uses the same types of interaction. There's little attempt to really explore the possibilities of multi-player. It's like every player creates their own little single-player farm/bakery/city/castle and you get to peek at each other's work and send network "pokes", gifts and requests. That is all you do. How's that really social?
At least MUDs or MMO's have a shared world, your avatars can interact, rather than lots of single-player games, only pretending to be linked together. There seems to be a real lack of creativity in the Facebook/flash game making community, they copy each others systems too much.
Maybe it's due to technical limitations of the facebook platform, but once proper MUDs/MMOs can run inside facebook, I think they will take off in a big way.