"Is this something impatient or nervous investors should be worried about?" said Herman Leung, analyst at Susquehanna International Group. "Yes, but for the long-term investors, I view this as an opportunity to potentially get a stock at a pretty good price."
Regulatory filings as soon as this week will disclose how many major shareholders sold their stock. Insiders are seen selling about 55 percent of the 1.2 billion shares that are set to come unlocked Nov. 14, according to an Aug. 9 research note by Brian Wieser at Pivotal Research.
Hedge funds that are using so-called short sales to benefit from the stock's decline may be active ahead of mid-November, said Jay Ritter, professor of finance at the University of Florida. These investors will probably sell shares with the aim of buying them back at a discount at a later date, he said.
"There will be selling pressure before the Nov. 14 lockup expires," said Ritter, based in Gainesville, Fla.
California-based Facebook declined to comment.
The shares rose 5 percent to close Monday at $20.01, after earlier slumping to an intraday record of $18.75. The trading volume was more than twice the average since the IPO, at about 100 million shares. On Aug. 17, 129.3 million shares traded hands, and the volume a day earlier, when the lockup expired, was 157.6 million.
By May, the number of shares on the market will increase 1.91 billion, compared with 421.2 million shares at the IPO.
Facebook's first earnings report as a public company last month also fueled concerns over how quickly it could draw revenue from mobile devices. Second-quarter revenue grew 32 percent from a year earlier, down from 45 percent in the first quarter.
Facebook's mobile advertising services are a key concern for investors as more users turn to tablets and smartphones. The company should benefit in the coming months from new tools and services that help companies market their wares to users on the go, said Scott Kessler, an analyst at S&P Capital IQ.
Two days before the lockup ended, Kessler had upgraded the shares to buy and said that annual revenue growth should be at least 25 percent for the next three years. He said, "The fundamentals are better than people think."