I'm not surprised that you feel this wasn't enough since it appears the reward is your motivation for finding an exploit in the first place. The reward shouldn't be viewed as payment for services rendered, but rather as a gesture of good will for performing your duty of responsible disclosure.
Without the disclosure of whitehat hackers, like I did, these exploits can also become available to dubious parties who could wreak (digital) havoc.
If you are truly a white hat then you aren't motivated by money. You are principally motivated by wanting to make the world a better, safer place. Since you're well-versed in computer security, that motivation will propel you to apply that knowledge to the web (or really to any vulnerability) when the opportunity presents itself.
If the reward is your motivation then at best this makes you a grey hat. You did the right thing, but for the wrong reasons. Note that for the following I'm not saying you personally will necessarily do this; but it would be easy from that position to start treating the group of people who you're willing to disclose a vulnerability to as a marketplace. Facebook will pay $4,500? Fair enough. L33tBotNetHaxz0r will pay $20,000? Done.
If you're someone who possesses the skill to discover security vulnerabilities, I think it's important to think through what your real motivations are. Is it for the rush capturing the flag? Is it for the money? Is it to help people? Your actions can affect many, so act wisely.
Edit: I should clarify that I mean that money can't be the principal motivation for a white hat hacker. I believe it's fine as a secondary motivation. I give an example in another comment below.