Ask HN: Could Facebook become the new PayPal?

Everybody's talking about advertisement as Facebook's main revenue stream, but it seems to me they're in a great position to take on PayPal and become the default payment system of the web.

It would be a tall order for sure, but if they did implement payments they would become number one overnight, since everybody already has a Facebook account.

Do you think this is realistic? Has Facebook ever talked about such a strategy?