You have to look at both acts of commission and omission when you're considering someone's impact on the world.
Zuckerberg is donating an eye-watering amount of money to charity. That's cool, but what about the rest of his wealth? His net worth is something like $14bn last time I checked.
That makes this a donation of around 3.5% of his net worth. No more impressive than someone with $200k of assets donating $7k to charity. In fact, given diminishing marginal returns to wealth, it's considerably less impressive. $500m is nothing to someone with a net worth of $14bn. It's just some digits on a piece of paper.
Consider a parallel universe in which Facebook never existed, and the $14bn that accrued to Zuck through his stake in the business was scattered amongst the many investors from whence it came. Who is to say these investors, between them, would not be donating an average of 3.5% of their net wealth to charity?