I disagree that this is "conceptually" a bank. Facebook's business model isn't lending out the reserves. If it was, banking regulation would apply.
Also, in a modern bank, reserves are maybe 10%, and you get less than zero percent interest after inflation for that risk exposure.
We've all seen in the past on how well "financial oversight" worked. At this point, I think I'd honestly rather put my money with Facebook than many of those actual banks out there. Facebook is at least a profitable company in its own right.