Facebook's initial public offering is shaping up to be one of the largest in history, with a price of $38 per share. The social network's valuation is hovering near $100 billion, but some financial analyst firms such as PrivCo believe Facebook is overvalued, in part because the company has yet to find a way to monetize its rapidly growing base of mobile users. Many early investors plan to cash out early, including Goldman Sachs, which might sell up to half of its Facebook stake. And CEO Zuckerberg? He plans to sell 6% of his shares.