Facebook's stock has fallen to $US19 for the first time, meaning it has lost half its market value since the company's initial public offering in May.
It hit that milestone on Friday, a day after the expiration of a lock-up period.
The end of the lock-up has provided some early investors and insiders with an opportunity to sell shares.
Investors have been concerned about Facebook's ability to increase revenue and make money from its growing mobile audience. Many analysts, however, hold positive opinions of the company's long-term prospects.
The stock closed on its first day barely above its initial public offering price of $US38. It has been below that level since.
On Friday, the stock fell 87 cents, or 4 per cent, to $US19 in afternoon trading.