At CSCL 2007 were one or two papers about group-interactions (connected to learning) in WoW guilds/forums. Unfortunately, I don't remember the author, but you might want to look into that.

Oh, and remains a little unclear to me: what exactly do you want to do? Describe the field of MMOs? Relate them to other social networks? If you haven't done so internally, try to specify your goal more closely. If you have done so: update your abstract :)