1 Number That Will Burn Facebook Investors

On Wednesday, Facebook shares changed hands on a private market at an implied valuation in excess of $100 billion. That's wonderful news for Facebook's employees and venture backers -- the only constituencies Mark Zuckerberg is concerned with in the impending initial public offering (the prospectus is explicit on this point). But if you're considering buying shares when the company goes public, it's awful news: The higher the pre-IPO valuation gets, the lower your expected share returns. . .