I definitely wouldn't use facebook as a bank account. Being able to use facebook to pay back friends and family when I owe them small amounts would be useful, though. As it is, I only keep cash around to pay people back for tickets they have bought, or if I am contributing to a gift for someone. If I could use facebook to transfer the money, I wouldn't have to keep cash around at all. If have used paypal for this a few times, but many people don't have a paypal account, or if they do, they have never used it to send money to someone they know.