Facebook's declining stock will face yet another test today when the first of the company's lockout periods to sell the stock will expire, freeing up some 271 million shares.

Insiders who hold stock in the company including Facebook COO Sheryl Sandberg and CFO David Ebersman will be free to sell the stock on August 16. The company's stock was down more than 46% from its IPO price yesterday morning when it traded for $20.38, so it's unclear how many will take the option. The expiration of a lockout period usually drags down a company's stock price a little further.

When LinkedIn and Groupon's first lockout period ended, the companies' stock prices fell 7% and 10% respectively.

This will be the first of many expiration periods for Facebook stock. A total of 1.8 billion shars could hit the market in the next year and half. Some 421 million shares have traded since Facebook went public in May. The next big dump could happen in November when Facebook converts restricted stock units held by many of its employees into actual stock.