This usually comes down to whether you need a FB id to sign into something. Its "Facebook" if you need to be logged into facebook to get to it. In this case,

This usually comes down to whether you need a FB id to sign into something. Its "Facebook" if you need to be logged into facebook to get to it. In this case, its a way to keep you on the Facebook site longer, instead of fracturing off another uninteroperable (inopreable, teroperable?) platform.