Facebook had A/B tested this tab bar feature earlier in the year for both iOS AND Android devices.
The first wave of Facebook beta users received the new tab bar controller layout from a server-side push which resulted in a gradual test base.
The reason I mention this is to deflect the assumption that this feature was added to preempt the release of iOS 7 which does indeed use a left to right swipe gesture to pop the navigation controller back one view.
This beta test was released for Android FIRST, which still utilized legacy menu button for Settings, Support and Log Out.
I believe the slide-in menu was becoming too much of a distraction, creating a barrier between what a user really wants to do within the app: News Feed, Messages, Notifications and then Account Info.