If they can move your money cheaper than credit and debit card networks. That is the only part I'm interested in. Given they are partnered with PayPal though, I'm not hopeful on it being low fee.
If Facebook has fees that are less than industry standard I would be highly suspect of that payment method.
It would definitely be a big shift from the "support" they offer now, admittedly, but if they can break the Visa/MC duopoly at scale, I think you could make a business case for it.
Again, I would be highly suspect of Facebook if they charged less than the industry standard, especially if they don't explain how they cut costs for risk management.
We'll see what their fee structure looks like.