AndroidPolice managed to get their hands on a system dump from the HTC Myst Facebook Phone. Most of the features are as you would expect, through there are some surprises packed inside. It makes use of a special Facebook app that isn't currently available anywhere else.
The system appears to be a mid-range HTC device that features a dual-core MSM8960 CPU, 1GB of RAM, a 720p 4.3-inch screen and a 5MP rear-facing camera. All of this hardware is powered by Android 4.1.2, through it appears it could be a heavily modified version of Android with deep Facebook integration.
The special build of the Facebook app bears a new package name as well as a host of new permissions. All of these new settings seem to be ones mainly used by home screen apps. HTC's own home screen app, "Rosie", isn't even installed on the device meaning Facebook's new app will likely take care of home screen functions.
There are also some hints that this version of Facebook might be released on the Google Play Store meaning that any Android device could replace their home screen with the Facebook version.