[mattj]

One big issue here (solvable, but something to think about): this would make messages a source for money laundering, which I'd bet fb would like to avoid.

Then again, they already deal with this with fb credits, so maybe this wouldn't be too taxing to deal with.

[wtvanhest]

If the percentage kept by facebook is higher than the cost of money laundering this won't be a factor.

For example, there are probably few people willing to launder money at a cost of 90%, and 10% seems like the amount FB would pay.

[jQueryIsAwesome]

The merchants would still need to use credit cards, so money laundering would not be an issue.