$4.5k is decent. Were you expecting to get rich? There are contests where you can rake in much larger amounts, and you could make a killing forming a penetration team if money is the issue.
Yes, 4,5k is decent. But put that in perspective of type of data and content Facebook exposed of its billion users.
And personally if Facebook's says "no maximum reward" (https://www.facebook.com/whitehat) I was expecting something different for a complete account access exploit.
Perhaps they didn't think it could be weaponized, so to speak. Accessing one account at a time isn't as big a deal as systematically accessing many accounts. Even if your exploit could do so, they may have safeguards against such robotic access.
And personally if Facebook's says "no maximum reward" (https://www.facebook.com/whitehat) I was expecting something different for a complete account access exploit.