Auto Updates are part of the Android store already, so this doesn't even offer new functionality. However, it does offer Facebook a couple interesting options.
1) They could force users to update, possibly to versions with more permissions. The keyword here is force, which the Play store won't do.
2) They could update devices that don't use the Play or Amazon store, if the application was somehow on the phone (from the carrier or manufacturer, perhaps). I think this is most likely.
3) They could remove most permissions from the Facebook app on the store, only to have the app request them after requiring an update.