Something a lot of people assume is that Facebook will eventually go into payment processing as an extension of managing your online identity. I personally think that would be cool. There's just one minor catch (nothing major over the long term) that might trip them up over the short term: Facebook Credits.
The idea was cool and it's a money-maker for Facebook accounting for some 10% of their revenues. But in order to go forward with any kind of payment processing, they will need to take a much smaller cut of the payment than the current Facebook Credits 30% cut.
If Facebook goes this route and offers something between a 2.5-5% transaction fee for purchases via Facebook Connect, there are two ways they can do this: If they simultaneously lower the Facebook Credits cut to 2.5-5% they will drop a large chunk of revenue going forward. Yes, in exchange for larger revenue returns over time, but in the short-term it looks like money being left on the table and may instill doubt in shareholders minds.
The other option is to keep Facebook Credits at the 30% cut and risk alienating that revenue source by pushing Zynga and its ilk to find a way to live without Facebook. (That nearly happened when the 30% Facebook Credit tax was originally instated, but this would be even more of a slap to the face, taking a 5% cut from retailers and a 30% cut from social gaming apps.)
They can definitely pull either strategy off, given time, but it'll be interesting to see which way they go on the issue, and how they spin it to make sure everyone is happy with their decision.