$100 million is rather less than 1% of Zuckerberg's wealth. Little old ladies donate higher percentages than this.
Donations, particularly ones which smell an awful lot like quid pro quo, don't count for much in my book unless they truly hurt. When Zuckerberg donates all but $100 million, I'll believe he's doing something other than buying his way out of hell. And when he donates all but $5 million, ideally anonymously, then I'll be impressed.
A man who built an empire and donated $100 million dollars vs. two douchey looking dudes who sued an empire. There is a difference (We are talking about how they got these gains)
Also, he just because he is worth a lot doesn't mean he has $10+ billion in liquid assets either.