Every time someone wealthy donates a whole lot of money to a philanthropic cause, people like you pop up in the comment section. "Oh it doesn't matter, it's only X% of his net worth, that's the same as me only donating Y". Seriously, you followed this template almost to the letter.
First, charity isn't a competition over who can sacrifice the most. At the end of the day, that $500,000 helps Wikipedia five thousand times more than your $100. More, probably, because his donation raises the profile and will convince others to donate. Second, people aren't "expected" to give anything, regardless of how much money they have.
I think the thing that annoys me the most is how obvious it is you came in here with preconceived anti-rich notions and then did mental gymnastics to convince yourself that you're "a better person" than this incredibly generous man. If you want to spread that kind of negativity around, go back to Slashdot.
I'm sure someone can articulate this better than I can, but seriously. These sorts of posts just piss me off.