It depends if we are talking about FB simply providing a service for taking payments similar to google checkout (using existing Cards) or whether you suggest that they try to replace card altogether.
In order to do that FB would be looking at setting up massive infrastructure and essentially having a "banking division" that would get bogged down in regulations and bureaucracy.
my point is, credit cards suck, and there is plenty of space for alternatives. There is a race in peer to peer and micro-payment space, and currently paypal is the leader, but it is far from perfect. I'm not a big fan of FB, but i think this article is on point, and they are well suited to enter this space. Being able to validate identity (thanks to all the information), and having a brand and cash to back it up. As for the new division, infrastructure, and bureaucracy, it would boil down to how well they can execute.
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.
In order to do that FB would be looking at setting up massive infrastructure and essentially having a "banking division" that would get bogged down in regulations and bureaucracy.