Facebook has enough users it could do anything; what would you do with a billion clients?
IMHO, I would become a credit card processor or bank of some sort, like a paypal; maybe even a Visa. Easily exchanged cash between members, everyone is on it, then every website will put a facebook login on their site, you would never have to register for another online shop again... you would pay with the credit card already stored on your facebook account. Stores would love it, since every time you purchased something it posted on your wall (depending on privacy settings), and you would get more honest reviews on products you purchase.