CEOs are still paying taxes on their non-salary compensation.
[1]: http://www.sec.gov/Archives/edgar/data/1288776/0001193125060...
I mean, think about it: Mark owns over $10 billion in Facebook stock--he makes over a billion dollars by pushing the stock price up 10% (or loses over a billion if the stock drops 10%). Either way, a $15 million salary would be fairly insignificant...
[1] http://www.businessweek.com/bwdaily/dnflash/content/may2007/...