Probably to receive stock options in compensation. Sergey Brin, Larry Page and Eric Schmidt have also done the same. They seem to be exempt from payroll taxes in the United Sates.
[1]: http://www.sec.gov/Archives/edgar/data/1288776/0001193125060...
[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/...