Facebook's stock market debut left founder and CEO Mark Zuckerberg with a paper fortune currently valued at $13 billion -- and a 2012 tax bill of around $1.1 billion.vZuckerberg's whopping tax hit stems from his move last May to increase his stake in Facebook. On the day of Facebook's initial public offering, Zuckerberg exercised a stock option and purchased 60 million Facebook shares at a "strike price" of 6 cents each.