Barker's reasoning behind declaring a Facebook MMO as a champion is due to the ease of playing with your friends. Facebook has the power to remove many of an MMO's social blocks, such as a lack veteran players, ease of access, a lack of servers, and the ease of playing with your friends, and turn those stoppages into forward momentum for a game and its community.
"Facebook doesn't care how long you've been on Facebook," Barker said. "Things like that show us that there are a lot of people out there that are interested in being involved in an internet community, especially with all the apps on Facebook, in playing casual games. At some point someone's going to do an MMO that really caters to that group, and is going to have a billion subscribers or whatever. There's another ceiling somewhere that we haven't hit yet."
While we're not entirely sure about the billion subscribers, (McDonalds of MMOs, anyone?) we can't say we don't doubt Facebook as a platform. As long as the polish on the game is present, of course.
[Edit: As sourced in the article from The Escapist, this interview was actually done with VideoGamer.com, not The Escapist.]