Zuckerberg and his lieutenants have tried in vain to reverse investor skepticism.
But they've been stymied by the difficulty in determining Facebook's actual value because the business models of social media companies are unproven.
Although Facebook has nearly a billion users, no one is sure whether advertisers can gain traction in selling products to them.
The challenge for Facebook's stock could worsen in coming months.
Facebook freed up about 271 million shares Thursday, adding to the 421 million the company sold in the IPO. An additional 192 million will be released in mid-October and a whopping 1.2 billion shares in mid-November.