You're making a mistake. There wasn't $14bn in investors pocket that was simply moved to Mark Zuckerberg. Mark Zuckerberg created wealth. It wasn't simply moving existing wealth.
In your parellel universe, there was less money to donate.
"You have to look at both acts of commission and omission when you're considering someone's impact on the world."
No, that's wrong. When considering someone's impact on the world, you only need to consider their impact. Mark's impact was creating Facebook, with all that implies, the good and the bad. And it also, now, includes giving 500 million dollars to charity, which will probably save at least some lives (doubtful it will have no impact or a negative impact, imo).
Unless by impact you mean something more like "considering whether someone is morally good or bad", in which case I'd agree with your statmenet, but still rule that Zuckerberg is good from the public info on him.