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.
The article itself goes on to discuss some statistics, for example, roughly 1 bazillion-ty people use social networks, and if each of them could pay a dollar for some content... you get the point. Also brought up is the general addictiveness of online gaming, and the fact that Second Life and the various games from company Nexon are already getting people to cough up real-life money for in-game items and content. All of this would sound very attractive to those in charge of the popular social networks, so perhaps those fetch-quests aren't too far off.