The current capital gains tax rate he would be avoiding is 20%, or $9b at today's Facebook market cap. There is also a 3.8% Medicare tax added on that would come to another $1.7B. The state of California would also get their hands on it and take another 13.3% (or about $6B).
tl;dr Giving capital gains away is very tax advantageous!
I did miss the Medicare Tax part yes, however he takes a $1 salary which would put his regular tax bracket at 10% and long term capital gains would be 0% to start and work his way up to 15%. As long as he doesn't cash out $400k a year he never pays that 20%, and he shouldn't need to ever. You also need to subtract IPO price from current price. Which is about going to drop those estimates about 35%. It still a big chunk of change, no doubt, but I have no problem with getting around it by going to charities. Especially when he already paid ~$2Billion in taxes for the stock at IPO time. The top 400 tax payers only payed $16Billion in 2009 so, he has paid a huge amount.
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!
tl;dr Giving capital gains away is very tax advantageous!