The popular Internet social networking site Facebook has lost nearly half its stock value, just three months after its widely heralded debut as a publicly traded firm.

Shares of the U.S.-based company were first offered to investors in May at $38, making the company worth about $100 billion, and its first-day share price reached $45. But the valuation has fallen steadily since then.

As early investors were permitted to sell their shares for the first time on Thursday, the Facebook stock fell to a new low of $19.87 a share, and the price dropped further to $19.10 in mid-day trading Friday. The current price erases about $50 billion of the company's value.

Facebook has more than 950 million users across the world who post pictures and information about their lives and join groups of people with common interests.

But analysts say investors are wary about Facebook's declining advertising sales. They say it has been slow to sell advertising on popular mobile devices, where about half of Facebook's users access the site. In one move to increase revenue, Facebook recently said it would offer online gambling to Facebook users in Britain.