If you say so. I'm sure somebody will buy anything. But the idea that there's an easily-tapped market for Facebook vulnerabilities, similar to the one for browser vulnerabilities? Like I said, I don't know. But I'm skeptical.
But that assumes the hacker is going to be his own fence. An arms dealer doesn't sell a gun on the basis of "yeah but if you jack 10 nice cars you'll make over $25K, so the gun is at least worth $4K".
Nonetheless, it just _feels_ that $4500 is a small amount, coming from such a large company. What is the downside of FB offering a higher reward, like, say, $20K? Or would the same argument apply, $20K is nothing compared to the millions tabloids would pay for a photo?
It's also not the case that this vulnerability == celebrity photos.