SAN FRANCISCO (Reuters) - Investors jumped on Facebook's regulatory filings on Wednesday for a glimpse into the company's inner workings ahead of one of the hottest initial public offerings in recent history. But there are also potential downsides. Here are a few key issues Facebook outlined as risk factors in its IPO prospectus: ** Facebook usage plateaus: Despite mind-boggling growth in recent years, the number of Internet users in the world is, after all, finite. ...