It would be shocking if he doesn't have alternate income from investments that would knock him up to the top tax bracket. Even moderate dividends on $25m add up to putting you in the top bracket. I have to imagine he has more than $25m in non-Facebook assets.

Regardless, he's saving a fortune in tax by donating money to his own foundation. It could turn out to be awesome like the Gates Foundation, but it could also be a decades long experiment in tossing money down the drain as he works on pet projects. Hopefully the former!