Let's examine the facts:
1. A Facebook vulnerability is dangerous to Facebook. A WordPress vulnerability is dangerous to a quarter of the internet. Facebook is not a high value target, relatively speaking.
2. A Facebook vulnerability will be patched once it is widely used. Facebook's security team is one of the strongest and most sophisticated of any company, and their processes would quickly catch this once it was used. The total impact of the bug would be negligible. You'd lose the ability to compromise accounts as soon as you tried to do it in any meaningful or lucrative way.
3. A vulnerability in Facebook might last a week before being patched, but a vulnerability in PHP will persist on the internet for years. No matter how many individual sites patch their servers, you'll still be able to pop a lonely server with social security numbers chugging along in a closet somewhere.
There really isn't much more to say about this. People claim bounties awarded by Facebook/Google/et al are undervalued every single time a bug bounty hits the front page of HN. Every single time, someone who is in the security industry patiently explains why it's not that valuable.
If someone tried to go to a blackhat group or go to the "black-market" (a shadowy, lucrative place that never seems to be very well-defined in these conversations), he would not even be able to find a seller, let alone one who would pay a lot.
What do you imagine someone would pay for this on the black-market? They'd need to profit from it. How much profit is worth their time?
Say they buy it for $20,000. Do you really think someone will derive $20,000 of profit from this before it's caught and patched by Facebook?
The only vulnerability worth $15,000 or more is one directly impacting a language, a widely used development library/framework or a widely used piece of software.
For further reading on bug bounty valuation:
https://news.ycombinator.com/item?id=7106953
https://news.ycombinator.com/item?id=9302188
https://news.ycombinator.com/item?id=9040855