My understanding is that they kind of do -- any game which allows in-game purchases must do so using facebook credits. You can set up a different "instance" of

My understanding is that they kind of do -- any game which allows in-game purchases must do so using facebook credits. You can set up a different "instance" of a game, but to allow interaction with the FB players, you have to tie it in to their currency. This means any emerging viral/social type game will be effectively locked in to facebook. (Those with clout, like Zynga, are able to negotiate better terms.)