wrt Zuckerberg you are mistaken. You are only liable for tax on a gain when you realize that gain. He became liable for tax when he sold stock and by selling it he had more than the cash needed to pay the tax.
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. Even if those shares are never sold, the IRS treats them as ordinary income at the time the options are exercised. The rationale is that such options are a form of compensation, just like regular wages.
1 - http://money.cnn.com/2013/03/28/technology/zuckerberg-tax-bi...