Facebook shares (Nasdaq: FB) declined the most in more than a month after Barron’s said the world’s largest social-networking service is overvalued and may drop to $15 a share or lower.

Facebook fell 6.3 percent to $21.41 in late-morning trading. Shares dropped as much as 6.8 percent earlier for the biggest intraday decline since Aug. 16.

Shares ended up down 9 percent, or $2.07, to close at $20.79 in heavy trading of nearly 79 million shares. The average daily trading volume is 55 million. 

Before today, the shares had slumped 40 percent since its May initial public offering.

Barron’s said Facebook looks overvalued as it falls behind in capitalizing on the growing number of mobile-device users.

Richard Greenfield, at analyst at BTIG Research, said Facebook’s mobile strategy makes him concerned, according to the newspaper.

The expiration of lockup periods for some employees to be able to sell 234 million shares by Oct. 29 and 777 million on Nov. 14 may keep the stock depressed, Barron’s said, without citing anyone.

Though Facebook’s stock has plunged since its May IPO, Andrew Bary at Barron’s said the stock trades at “high multiples of both sales and earnings, even as uncertainty about the outlook for its business grows.”

At issue is the shift of Facebook’s massive user base to mobile devices, where Facebook is still figuring out advertising. Bary said success in the mobile space is “no sure thing” for the company. Mobile ads must fit into much smaller screens, which doesn’t give Facebook “much room to configure ads without alienating users,” Bary said.

Facebook also has what Bary called “significant” stock-based compensation expenses. Last year, the company issued $1.4 billion worth of restricted stock and $1 billion so far this year, he noted. Yet technology companies such as Facebook “routinely encourage analysts to ignore stock-based compensation expense – and most comply. This dubious approach to calculating profits is based on the idea that only cash expenses matter,” Bary wrote. “That’s a fiction, pure and simple.”

Bary said he thinks Facebook’s stock is worth $15, well below its current price even with Monday’s drop.

“That would be roughly 24 times projected 2013 profit and six times estimated 2013 revenue of $6 billion, still no bargain price,” he wrote.

A representative for Facebook could not immediately be reached for comment Monday morning.

Last week, research firm eMarketer said it expects Google Inc. to surpass Facebook in U.S. display advertising revenue this year. In February, eMarketer predicted Facebook would stay ahead of Google. The social networking company had surpassed Google in 2011. But Facebook’s ad revenue has fallen short of the expectations eMarketer set in February.