Facebook has filed to trade publicly. The social networking company filed paperwork Wednesday. The company hopes to raise $5 billion in its I.P.O. Because it is a privately held company, it hasn't had to release its financial statements. That means, there's no telling exactly how much its stock will be worth on the open market. But based in estimates, its reported I.P.O. is expected to be one of the biggest in U.S. history.