On the other hand, 2-3% of that $4 trillion is being consumed by the payment processing chain. Only a portion of it goes to Visa; the rest presumably go to banks and other layers of the service. Facebook potentially could, by virtue of being a trusted holder of identity worldwide, subsume all layers of this chain.
Look at it from this perspective: It currently "costs" 2-3% of a transaction to make a CC purchase. That's a huge dead weight loss. Is Facebook capable of using its brand, reach, and identity database to create a more efficient system?