The Facebook slide on Wall Street continued Thursday amid fears that early investors eligible to sell their shares would create a surplus on the market. Shares fell more than 6 percent, closing at $19.87 — its lowest close since the initial public offering in May.

The company's widely anticipated public debut seems to have been jinxed from the start, and for a variety of reasons, including technical problems on Nasdaq and slowing sales growth. Complicating its recovery is the prospect of about 2 billion shares that early investors and employees will be able to sell beginning this week and continuing through May.

The first and smallest opening in that gate was Thursday, when about 271 million shares were eligible to be sold. The shares are held largely by early investors, like Accel Partners and Goldman Sachs. It is unclear whether they were sold Thursday; the transactions are likely to take some time.

But trading volume in the stock was high: 157 million shares, versus a 30-day average of 31 million.

Companies that go public typically compel insiders to hold on to their stock options for a period of time, to prevent the market from being swamped with surplus shares. When the lockup period, as it is known, expires, it can typically weigh on value. That is especially painful for Facebook, because its value has already fallen so precipitously and unexpectedly. The largest tranche of shares are eligible to come on the market in November, so Facebook could face a deflated stock price for some time to come.

The lockup expiration adds pressure to acute difficulties already facing the company, which made its Wall Street debut with a $100 billion valuation barely three months ago.

The low stock price complicates Facebook's ability to attract and retain employees. Without prospects of the stock rising rapidly, prospective employees may choose to work at hot startups where they can foresee stock gains that would supplement their salary.

For its stock price to go up, Facebook has to convince Wall Street analysts and investors that the personal data its 955 million users share about themselves can be better used to make money. Despite the fact that Facebook has information about a user's friends, habits and photos, advertisers are not convinced that Facebook ads are more effective than online ads appearing elsewhere.

So far, its revenues come largely from advertising and from proceeds of virtual games that people pay to play on the Facebook platform. On both counts, Facebook has struggled, as the company reported slower sales in its earnings report in late July. Its users are increasingly logging in to their accounts on mobile devices, where Facebook has only recently — and cautiously — started selling advertisements. And its vital partner, the social-games developer Zynga, has hit speed bumps of its own and its shares have plunged.

"All the lockup is doing is enabling people to sell," said Richard Greenfield, a media analyst with BTIG, a brokerage firm. "The issue is still confidence in Facebook's transition from the PC to mobile."

Greenfield was among those who advised against buying Facebook shares during its initial public offering. He still advises against it.

Some investors are still smarting because many of the company's early big backers — including Accel Partners, one of its first venture capital investors — sold a hefty portion of their shares at the peak price in May.

Mark Zuckerberg, a co-founder and chief executive, sold a portion of his shares in the offering — to meet his tax bills, the company said.

The 28-year-old's net worth fell to $10.2 billion, its lowest point since the company's May 17 initial public offering, according to the Bloomberg Billionaires Index.

All told, early backers sold more than $9 billion in shares, although they still own significant amounts.

Some of those early backers — though not Zuckerberg — were eligible to sell their shares starting Thursday.

Investors who remain confident in the company's future point out that it is still profitable, with a potential to become a lucrative advertising platform.

Facebook is experimenting with new ways to make money, including creating new advertising tools and offering online gambling to its users in Britain.