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!
And either way, the charitable organizations deserve to be supported and will do good for society, and they get to pick how it is used. Which doesn't seem like a bad thing, when the Federal government can barely agree on any budget at all.