This isn't directed at Zuck or Facebook, or anything. Because I really don't know him.
But regarding your shock about the dollar amount...
Keep in mind that it's incredibly easy to be generous when you are at the top. It's actually so easy, that it's almost effortless.
There really is a difference between a man that writes a check that makes no difference to him (in the amount), and a man with nothing that roles up his sleeves, goes out, and dedicates himself to helping others.
I'll celebrate the latter before I'll celebrate the former.
One throws money away, the other helps change everyone around him.