Shares of Facebook Inc. (Nasdaq: FB), the world's No. 1 online social networking website, fell Wednesday on volume that doubled typical daily levels, as many employees got their first chance to sell 234 million shares that had been locked up after Facebook’s initial public offering in May.

Facebook’s stock closed down 83 cents, or 3.79 percent, to $21.11 in Wednesday’s session. For five agonizing months, Facebook’s employees watched helplessly as the stock of Facebook plummeted in value, while the big guys dumped their pre-IPO shares for $38 each. The stock is now trading at roughly 45 percent below its IPO price of $38.

The lockup was supposed to expire on Monday for 179 million employee Facebook shares, but shareholders could not sell until Wednesday because U.S. stock markets were closed for two days as a result of Hurricane Sandy. Lockups are designed to prevent a stock from experiencing the kind of volatility that might occur if too many shareholders decide to sell at once.

Facebook, with roughly 1 billion users, experienced huge demand for its shares when it was a private company and became the only U.S. company to debut with a market value of more than $100 billion.

The stock hit a record low in August when early investors started to cash out. Peter Thiel, a former PayPal co-founder and an early Facebook investor, sold a majority of his Facebook stock at roughly $20. The large sale by Thiel, who also sits on Facebook's board, spooked investors who interpreted it as a sign that insiders were losing faith in the company.

Wednesday's unlocking is just an opening act. The next big test for Facebook’s stock price will be on Nov. 14, when 777 million more shares and stock options held by employees can be sold. Additional lockups expire in mid-December and May 2013.

Facebook's 28-year-old Chief Executive Officer, Mark Zuckerberg, has already said that he won’t be selling stock until at least next September.

Last week, Facebook reported a 32 percent quarterly revenue gain, to $1.26 billion, slightly topping Wall Street's expectations. Investors were encouraged by the fact that mobile ads generated 14 percent of the company's advertising revenue.