So it looks like Facebook are going announce their entry into the mobile world this week.
At the moment it looks like they're either going to announce a fork of Android or, as John Gruber suggested, maybe some sort of launcher/home screen replacement for existing Android phones. Since it seems HTC are involved and given Facebook's aspirations I would think it's the former.
This kind of explains Facebook's recent push to get free calls into their existing Facebook Messenger apps. Starting their own mobile platform with this capability baked in from the beginning would make a lot of sense and probably be attractive to a lot of people.
Personally, the thought of every single thing I do on my phone being seen and catalogued by Facebook isn't very appealing.