Earlier quoted context omitted.
Facebook's potential advantage in this space is not that they have a billion users, it's that they arbitrate the identity of those users. And payment is inexorably linked with identity, especially WRT avoiding fraud. Certainly it is a difficult market to enter, but a few billion dollars can buy you a lot of expertise, relationships, and time to build experience. The reason Facebook should go into transactions is the…
The problem is that it isn't where the money is, not much more money than Facebook is already making anyway. Not unless facebook intends to commit theft. The money today is in selling digital goods which have a near zero transactional cost. Music, movies, games, books, etc. Not that it makes sense for facebook to get into that business either, necessarily.
Granted, a worldwide payment scheme is a Very Hard Problem To Solve for many nontechnical reasons... but this is exactly the kind of problem that large, ambitious companies with lots of street cred should try to attack. If it was easy, everyone would be doing it.