How long before Facebook will attack paypal on its home turf?
They could do a pretty good job of it too, what with facebook knowing who you are (within reason) and who your friends are they could set this up in such a way that they could side-step the biggest problems with payments through paypal.
The tell-tale will be when facebook registers as a bank somewhere.
Edit: hello downvoter, feel free to disagree a bit more verbosely.