Today, @evleaks, an anonymous mobile leaker, posted photos of what it's going to look like at 9to5Google. Facebook's modification of Android appears to make the operating system simpler.
By most accounts, Facebook isn't totally taking over Android. It's just providing a "skin," or its own layer of design atop the operating system. This is widespread with Android. HTC and Samsung each have their own tweaks to Android. HTC has "Sense" and Samsung has "TouchWiz".
This is just going to be the Facebook "Home" or something like that.
Yesterday, @evleaks published a photo of the HTC First, which will be the new phone Facebook uses to demonstrate the new Android software. According to the leaked specs, it'll be a mid-range phone with relatively weak specs.