Another 800 million shares of Facebook become available today as early investors and employees are now eligible to sell their shares.  Facebook had implemented five staggered lock-up dates, when early investors can sell their shares.  Lock-ups are put in place to protect the company from having too many shares hit the market immediately following an IPO.  This latest lockup expiration, the third and largest, could potentially flood the market with shares, negatively impacting the stock price.  As it stands, Facebook has lost almost half its value from the initial $38 opening price.  The first lock-up expiration occurred back in January when 270 million share became available to sell and the stock dropped six percent.  The second expiration was two weeks ago, when employees were allowed to sell 234 million shares.  The next expiration is December 14 and the last is May 18 when 147 million and 47 million shares respectively can potentially hit the market.  There was some good news regarding Facebook when third quarter results showed increasing ad sales and efforts to grow the mobile user market.  The stock increased nineteen percent in response but has since come down as the market sagged following the Presidential election.