The falling price of Facebook's stock is seen on a screen reflected in the window of the Nasdaq building in New York August 16, 2012. (Reuters/LUCAS JACKSON)

SAN FRANCISCO - Facebook Inc shares sank as much 4.3 percent on Friday to set a new low, a day after early investors got the green light to sell for the first time.

More than 270 million shares owned by the early investors became available for trade on Thursday after a 3-month curb on sales ended. That's more than one-half the 421 million shares sold in its initial public offering on May 18.

Facebook hit a low of $19.01 Friday morning and were down almost exactly 50 percent from the $38 offering price at its May IPO. At midday, the stock had eased to $19.15, off 3.5 percent.

Nearly 62 million shares of Facebook traded hands in the first two hours of trading on Friday.

More than 1.4 billion additional shares held by early investors and Facebook employees are set to become available for trading by year's end, adding further pressure on shares of the once high-flyer.

Facebook, the world's No. 1 Internet social network founded by Mark Zuckerberg in his Harvard University dorm room, became the only U.S. company to debut with a market value of more than $100 billion.

But investors have grown disillusioned with Facebook's inability to articulate a plan to reverse slowing revenue growth.