There may be a lot more Facebook millionaires soon. The social-media company submitted paperwork to regulators for a $5 billion initial public offering, one of the largest U.S. market debuts in history. (Google’s IPO was $2 billion.) Facebook said in the filing that it made $3.71 billion in 2011. The IPO haul could raise founder Mark Zuckerberg's net worth from $21 billion to $28 billion. The filing also said Zuckerberg will sell shares in the IPO, though it didn't specify how many, and use the proceeds to pay taxes.