The stock closed down 4.1% at $19.05 and fell to $19 for the first time, meaning it has lost half its market value since the company's initial public offering in May.
The milestone came a day after some early investors and insiders had their first opportunity to sell shares following a 'lock-up' period.
Investors have been concerned about Facebook's ability to increase revenue and make money from its growing mobile audience, although many analysts still hold positive opinions of the company's long-term prospects.
Following the social media network's notorious initial public offering (IPO) in May, the lock-up period that forced pre-float investors to keep hold of shares ended on Thursday, suddenly placing millions of shares on the market.
When Facebook went public three months ago, 421 million shares were issued at $38 (£24.20) each, rising as high as $45 (£28.66) each after trading began.
But the stock closed on its first day barely above its initial public offering price of $38 and has been below that level since.
Analysts are now questioning the moves shareholders, including Goldman Sachs (NYSE: GS - news) and Microsoft (NasdaqGS: MSFT - news) , will make - testing their long-term investment commitments to the social networking site.
But Steve Place, a founder of options analytics firm investingwithoptions.com, said the drop in Facebook's share price could be a good thing for the company.
He said: "If Facebook was trading at $30 (£19.11), we would see a much larger effect from the lockup expiry. But at $20 (£12.74)? Not so much."
Mr Place explained that investors may choose to hold onto their stock in the hope its value will rebound and head back up.
Facebook's chief executive Mark Zuckerberg is obliged to keep hold of his stock until November 14.
It comes as reports have emerged that advertisers may soon be able target Facebook users more directly by posting adverts onto their news feeds.
Currently companies only have the ability to advertise products or brands to users who have 'liked' their pages.
Facebook is believed to be testing the scheme which, if successful, would open advertisers up to the site's one billion users, who have not necessarily become fans of particular brand pages.
Paul Armstrong, head of social at media agency Mindshare UK said: "Facebook have to walk a fine line now as they have to keep both stakeholders and users happy.
"Mark Zuckerberg has a colourful past with user privacy and has a clear vision for a more open future web for all.
"It is important to remember this is still only a test at the moment and Facebook has a history of reversing decisions made on these subjects when users stand up in large numbers."
Mr Armstrong added: "It is early days still after Facebook floated - those expecting a quick windfall have been disappointed but Facebook are in for the long haul as brands should be."
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