The stock lost 83 cents, or 3.8 per cent, to close Wednesday at $20.88. It is 45 per cent below its initial public offering price of $38.
Shares briefly hit $20.84 during the day. The previous low was $21.61, reached Tuesday.
The stock has been falling since Facebook released quarterly earnings last Thursday for the first time as a public company.
Investors were disappointed despite second-quarter results meeting Wall Street expectations, with revenue one-third higher than last year.
Facebook began trading publicly in mid-May following one of the most anticipated stock offerings in history.
But since then, investors have been concerned about its ability to keep increasing revenue and make money from its growing mobile audience. Many analysts hold positive long-term views.