He stumbled around a bit trying to work out how to help, but he brought a flaw to your attention in what he thought was a polite way. If unleashed, this bug could've been used to wreak havoc on Facebook and damage the company's reputation. $500 is the very least FB should be paying.
If they bothered to look at his profile (it's public), they'd see he looks to be a great fan and tinkerer on the Facebook platform.
Suppose I hacked into a bank and stole money from some account. Would the person whose account was hacked be able to have some legal recourse against me? I'd imagine it would be the bank.
If this is the case, then surely facebook could just choose not to press charges, and if so, what would be unlawful paying him in that case?
They might even throw a little fanfare his/her way to send a message that the bank appreciates being told and not robbed blind. (Especially given that they're a "community bank" built by pioneers and not a monolithic marble statue institution :-P)