Even though companies like Facebook, LinkedIn, Twitter, and Zynga are private, you can still buy stock in them. The U.S. Securities and Exchange Commission (SEC) is now investigating the background of this private stock trading and is conducting investigations. The SEC sent out requests for information from the companies that facilitate the private company stock trading.

In a SecondMarket share auction that took place last week, the end price per share was $20.76 for Facebook. Facebook has about 2.5 billion shares outstanding after a 5-1 stock split earlier this year. Based on the price per share and the outstanding shares, this puts Facebook’s valuation at about $50 billion. This means that Facebook is worth more than Yahoo! and eBay and their value is about 1/4th of Google and Microsoft. Below is an e-mail sent out to buyers on SecondMarket: