(Menlo Park, CA)  --  Facebook CEO Mark Zuckerberg's wallet may be a little lighter following a billion-dollar tax bill related to the social network's initial public offering. 

He bought 60 million options on the first day of the IPO at a rock bottom price. 

The IRS treats the value of those shares as income whether they're sold or not.  Current state and federal tax rates pushed that bill to more than the billion-dollar mark.