Black hat markets, presumably. At least that is the point being made by commenters here. Granted, selling the vulnerability is illegal and immoral, but that doesn't stop it from happening. The 'market rate' for vulnerabilities seems to be higher than what Facebook and Google are paying out.
This makes sense economically to me. In order for it to be worthwhile for a vulnerability discoverer to sell the exploit, the reward should overcome the cost. In this case, the cost is the probability of getting caught multiplied by the severity of the punishment.
Really? That's exactly what you're doing with Facebook - selling a vulnerability to them, which they then pay you for. So, disclosing to some third party ought also to be fine. The morality or otherwise is up to you though, I guess...
EDIT - I just read @tptacek's reply below. I guess that selling to known criminals, with the knowledge they would use the exploit to commit a crime, _is_ going to be illegal most places.