29 March 2013New York Times:
For instance, when the device is turned on, it will immediately display a Facebook user’s home screen, the source said, a fact reported earlier by The Wall Street Journal. Facebook’s camera and messaging apps will be the default apps for the core functions of the phone, the Facebook employee also said.
Seeing as Facebook already has a cache of Android apps and Android developers at their disposal, they might as well experiment with a fork of Android. The additional resource investment required is almost negligible.