Facebook is abusing the good will of white hats by offering such trivial sums, and they're reducing the security of their platform in the process. They have how many $100k+ engineers who couldn't find this? And how much does the average security breach cost per record, $100-$200? This exploit alone could have exposed them to millions in losses at that cost.
This is what turns white hats into black hats, and I wouldn't blame the guy for selling his next exploit rather than disclosing it. A famous guy once said "we create our own demons". And then the guy in Iron Man 3 said it. And now I'm saying it.