Shares of Facebook (Nasdaq: FB), the No. 1 social network, set a new low of $26.44 on Monday after the influential research firm Bernstein started coverage with an underperform rating.

Bernstein suggested a target price of only $25. Analyst Carlos Kirjner projected 2013 Facebook revenue of only $6.4 bilion, about $100 million shy of the consensus, because advertising growth will rise only 38 percent this year and 32 percent in 2013.

As a result Facebook shares closed down 82 cents, or 3 percent, at $26.90 on Monday. That's the lowest close for the social network site since the IPO on May 17.

On Friday, Facebook shares fell 7.5 percent.

Facebook's continued fall came despite reports it may be near a settlement with Yahoo (Nasdaq: YHOO), the No. 3 search engine over patent litigation, as well as that it might start to offer service to members under 13. That would allow it to keep expanding its members from the 901 million reported as of March 31.

Other social-networking stocks fell Monday, including Zynga (Nasdaq: ZNGA), the game site that relies upon Facebook. Its shares fell 5 percent to $5.71, and Jive Software (Nasdaq: JIVE), the business social networking site, whose shares fell 4.4 percent to $14.74.

Facebook, of Menlo Park, Calif., was priced at $38 by its underwriters led by Morgan Stanley (NYSE: MS). Since the IPO, shares have lost more than 29 percent of their value, trimming the company's market capitalization from $104 billion to only $58.3 billion.

The value of CEO Mark Zuckerberg's personal stake has plunged from $19 billion to only $13.5 billion.

Facebook, though, raised $16 billion in its IPO.

Bernstein, a unit of Alliance Capital Management, provides research only to Wall Street investors and is well-regarded because it doesn't engage in other activities.

Morgan Stanley fell 37 cents to $12.36.