MILESTONE: Facebook's early investors and some directors become eligible on Thursday to sell stock they own in the social networking company. Many Facebook employees receive the same right to sell their shares later.

SIGNIFICANCE: It's conceivable none of them will sell. But if they do, up to 1.91 billion more shares could flood the stock market. If too many people sell, Facebook's stock price could decline. It's already down 46 percent from its initial public offering price.

LIKELIHOOD: Wedbush analyst Michael Pachter believes it's unlikely that top executives will sell as soon as they can, as it would look bad.