Shares of Facebook (Nasdaq: FB), the No. 1 social networking site, plunged as much as 11 percent on Monday after critical comments were published in Barron’s magazine on Saturday suggesting shares weren’t more than $15.

In late trading, Facebook shares were at $21.07, down $1.80 or nearly 8 percent after tumbling as low as $20.36 earlier. Since the Menlo Park, Calif., company went public in May at $38, its shares have traded as low as $17.55.

Barrons, a weekly magazine published by Rupert Murdoch’s News Corp. (Nasdaq: NWS), suggested Facebook faces continued challenges in the advertising space as well as problems from the huge number of options granted to employees which become exercisable over different periods.

Rather than the potential 1.88 billion shares Facebook lists in current filings with the U.S. Securities and Exchange Commission, Barron’s said there are more likely 2.65 billion. As their owners become eligible to receive and sell them, there’s likely to be more pressure on the share price.

As well, the magazine argued Facebook ought to list issuing stock options as an expense like many other technology companies. Not doing so hides potential costs.

Facebook’s price-earnings ratio is a hefty 117, especially because the company reported a big second-quarter loss. By contrast, Apple (Nasdaq: AAPL), the world's most valuable technology company, has a P/E ratio of 16.2 and Google (Nasdaq: GOOG), the No. 1 search engine, has a P/E of 22.2.

“Facebook is valued at $61 billion, or $53 billion excluding its estimated $8 billion in cash.” Barron’s said. “That’s more than 10 times estimated 2012 revenue of $5 billion. Google trades for half that valuation.”

The Facebook fall-off isn’t the first time a critical Barron’s story has affected a stock. On Mondays, the financial weekly’s picks and pans often have great market influence.