They would most likely still have to deal with the credit card cartel in one way or another. Simply putting a nice payment front end up doesn't make VISA suck any less.
Another advantage of credit cards is the credit part and it would seem a big stretch for FB to get into the lending game.
I think how well they can execute is only one part of the puzzle, since they are going to have to deal with the governments of multiple jurisdictions and big financial institutions and will basically be at their mercy.