Just because you aren't slaying bandits and completing fetch-quests when you're on Facebook doesn't mean it isn't an MMO. Or at least that's what Gabe Zichermann has asserted in a USATODAY.com article on social networks and online games. Zichermann is making a game called "rmbr", which will be played by tagging and sharing photos.
A quote from Zichermann on the issue: "The reason why Facebook is a really compelling MMO is because it's fun and you get something out of it". That sounds pretty sketchy, but if you think about what MMOG means, these social networks like Facebook and Myspace do almost have the bases covered -- they have a large number of people online and interacting with each other -- all that's left to satisfy is the "game" part, giving people more things to do essentially, and with applications like Scrabulous already incorporated in to Facebook, it seems like only a matter of time before this happens.