I don’t think a Facebook phone is any kind of answer. It will have to be super-cheap to compete. After all, why buy a cow (a Facebook phone) when you can get milk (a Facebook app) for free? A Facebook phone looks like an expensive distraction, a war of choice.
I don’t understand how a Facebook phone would be an appealing choice. They would have to make it extremely cheap, and completely ruin the Facebook app experience for every other platform.