This is a little bit of a leading argument but it seems to sum up external arguments.
I still have yet to rediscover where I saw that Facebook was filing to be a bank.
$2billion in cash can lend out $18billion in value.
@ 5% interest that's $900 million a year in profit.
@ 10% that's $1.8billion a year.
& that's just on interest.
Facebook makes transactional profits too, so on $18billion in credits, I bet you could see $100 billion in transactions if they one day allowed peer to peer.
Visa has about $2-3billion a quarter in revenue via their transaction fees.