Facebook’s stock plunged to an all-time low today as the market braced for the company’s insiders to dump their stock after the expiration of a lock-up period.

Investors ranging from Accel Partners to Goldman Sachs, Zynga CEO Mark Pincus and Facebook board members James Breyer, Peter Thiel and Reid Hoffman are among those free to sell stock they own now.

By 10:30 a.m., the price was down 7% to $19.69 a share. By noon, it had climbed back to $19.95 a share. In May, the initial public offering of the stock was $38.

If the stock hits $19, it will have lost half its value since Facebook went public in May.

Before noon, the company lost $4 billion in market value, due to the plunging prices.

Facebook's stock plunged to an all-time low today as the market braced for the company's insiders to dump their stock after the expiration of a lock-up period

Mark Zuckerberg himself lost an estimated $1 billion Thursday morning alone.

It’s not yet known whether anyone had sold shares. The stock price decline could reflect investors’ anticipation of such a move.

More than 270 million shares have been unlocked – more than one-half of the 421 million shares sold in the May initial public offering of the social networking company.

Roughly 64 million shares of Facebook traded hands in the first hour of trading, more than double its 50-day daily average of just under 30 million shares.

“Pressure will be back on the shares now that liquidity is back in the market,” said Frank Davis, director of sales and trading at LEK Securities in New York.

“If (the value of) your holdings has been cut in half, are you going to sit around and risk the rest of that?”

It’s conceivable no one would sell those extra shares, but if too many do, Facebook’s stock would decline even more because the market would be flooded with additional shares.

It’s been a rough run for Facebook. After one of the most-anticipated IPOs in history, Facebook suffered what may be the most-botched public offering as trading glitches marred its first day.

Facebook, the world’s No. 1 Internet social network with 955 million users, has seen its shares pummeled since the market debut in May that put its value at more than $100 billion.

Investors have been concerned about Facebook’s ability to keep increasing revenue and make money from its growing mobile audience, even as many analysts hold positive long-term views.

Those eligible to sell additional shares Thursday were the investors and directors who had participated in the May IPO. The exception was CEO Mark Zuckerberg, who will be ineligible until November.

Other shareholders, including many Facebook employees, will be able to sell beginning in October. The last lockup period expires next May, a year after the IPO.

In all, up to 1.91 billion more shares could flood the stock market over the next several months – more than four times the 421 million shares that have been trading since Facebook’s IPO. Of the 1.91 billion, 271 million shares became eligible for sale Thursday.

What are lock-ups?

Lock-ups prevent company insiders from selling their shares in a newly-floated firm.

They usually start to expire 90 days from the initial public offering (IPO).

They are designed to prevent the share price from fluctuating wildly if too many investors decide to sell their shares all at once.

The lockup expired for initial investors on Thursday. It expires for Facebook employees in October.

CEO Mark Zuckerberg must keep his shares until November.