When the going gets tough, the investors get antsy. Facebook’s stock hit an all-time low in Thursday morning trading as a lock-up period on the company’s IPO ran out, giving investors an opportunity to ditch out on their shares. The social-media giant’s stock fell 7 percent at the opening bell to $19.71. If it sinks to $19, its value would have halved since Facebook went public in May. The company’s much-anticipated IPO ended up being one of the most disappointing in history due to trading glitches, and things haven’t improved much since. A total of 271 million shares became eligible for sale on Thursday, in addition to the 421 already trading.