Everyone else seems to be writing about the Facebook IPO so I will reluctantly follow the herd, but from an estate tax perspective.

Zuckerberg and other FB founders have the ability to avoid future gift and estate taxes on their FB stock by creating a GRAT.  That trust allows them to annually receive income generated by the stock, but ultimately transfer it to their heirs at the current  value discounted for the time value of money.  This strategy also works for people who are not only founders of social media behemoths.