Okay, but still. It seems like a stretch that the guy is secretly evil but voluntarily gave up almost $1 billion to deceptively prove he's not actually a bad guy. You can claim to be a good guy and wait a few months to cash out - leaving money on the table IS a real signal, even if he's already rich.
Depends on if he actually left money on the table or if he speculated that there would be more money coming to him if he publicly left FB when he did.
Social/public good will is a kind of money itself that can't easily be measured in dollars. That's a big part of the reason extremely wealthy people engage in philanthropy.
Good will is a currency that opens some doors that no amount of raw dollars can open.