What is a retail bank but some product ledgers and user identities? Banks understand that they are rapidly being reduced to artifacts of regulation. The ones I have spoken to have a primary mandate to "preserve the business model."
The identity information FB has is often more reliable and valuable than the documents governments provide. Perhaps to coin a phrase, what FB and other social platforms have is "collateralized identity," where your relationships and personal behavior data are the collateral on the assertion of identity. The documented one relatively is just a "registered identity."
Difference is the amount of friction, authentication, and recourse you have to each of them. A registered identity is mainly for legal recourse (tax, travel, and law enforcement.) What FB has might lack a bit of recourse, but it has anti-gravity frictionlessness, and a high degree of authentication.
The point at which an institution becomes indifferent to the level of recourse in the identity and switches to one with less friction, more data, and more authentication - looks pretty similar. In fact, the ones who demand registered identity are at a structural disadvantage to orgs who provide services using collatoralized identity.
There is very little to keep FB from becoming a global bank, and current banks will have the option to participate, or compete against the ones who do. It's strategic in so many ways. I don't think this is a normal business arrangement. It may more resemble offering terms on which FB will accept the banks' surrender.