Facebook have just announced "Facebook Home". It's not a new phone or a customised version of the Android OS, it's essentially a launcher replacement which completely changes the way your Android phone looks.
Facebook Home integrates very deeply into your Android experience with a number of features such as Cover Feed, Chat Heads and a customised app launcher.
The Cover Feed are updates from your friends which fill the whole screen. It's the posts you would usually see on your news feed. It shows photos, status updates, links and much more.
Chat Heads is a feature which keeps you always connected to your Facebook messages. When you get a message it'll pop up, showing the messagers profile pic. It enables you to stay in the app you're in, but reply to a friend quickly if you like. You can of course hide this, it would get highly annoying if it were on all the time.
Facebook Home includes its own app launcher. At the top you now get three buttons which enable you to quickly post content to your Facebook profile. You can then choose what apps show up when you open the launcher. You can still access all your apps by simply swiping to the right - you'll be given the full app drawer.
Facebook Home will be available on Android on the 12th of April, directly from the Google Play Store. Only a handful of devices will be supported at first, so don't expect your device to be supported at launch. Android tablets will also not be supported at launch, Facebook haven't given a definite date to when it'll be available on tablets, just saying it'll be coming in a "few months".