SharesPost is an online marketplace where you can buy and sell stock in private companies. SharePost has raised $3.3 million and moved their headquarters to northern California. SharesPost was originally based in Santa Monica and was founded by Greg Brogger. The investors were undisclosed. [SEC filing]

On the secondary stock trading market website SharesPost, Twitter has hit a $4 billion valuation. This is a $300 million increase in less than a month. In December, Twitter had a valuation of $3.7 billion. This trade is based on a smaller set of Twitter stock and it is not the official internal valuation. On SharesPost, Facebook’s valuation is trading at between a $50-$75 billion valuation.

LinkedIn has hit a $3 billion valuation in private funding in a recent trade on SharesPost. SharesPost sells shares of private companies as a secondary market. When SharesPost sold shares in LinkedIn at $30 per share, it gave the company a valuation of $2.91 billion. About 95,000 shares were sold at that price.

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.