Specifically, Facebook wants to integrate deeply into the contacts list and other core functions of the phone. It can only do that if it controls the operating system.
I have no doubt that they want this information and more. I have no doubt they have thought about and are exploring what they could do with their own phone platform. I do doubt that there is a huge interest in a Facebook phone and further doubt that one would be better than the likes of Windows Mobile let alone iOS. I am however fully prepared to eat those words if needed.