The social network will unlock about 777 mil shares of its stock on Wed., roughly one-third of outstanding shares and the largest block of Facebook (FB) shares available since its May IPO. The shares are mostly owned by employees or former employees, granted as part of their compensation prior to the IPO. Expiration of 2 prior lockups triggered sell-offs. Unlocking them could further pressure its stock price, which has fallen sharply since the IPO. Facebook, which went public at 38, fell 1.1% to 19.86.