Facebook has filed papers for what's expected to be the largest initial public offering ever to come out of Silicon Valley and one of the largest in U.S. history. Ending months of breathless speculation, the 8-year-old social networking company has submitted registration documents with the U.S. Securities and Exchange Commission that set preliminary goal of $5 billion. Facebook had discussed raising as much as $10 billion. Final pricing will not be set for months, and the size of the IPO probably will increase with investor demand. The filing sets the stage for an IPO in May.