Shares of Facebook (Nasdaq: FB), the No. 1 social network, plunged as much as 7.5 percent in Friday trading, reversing Thursday's 5 percent gains. Overall, markets fell about 2.5 percent on economic fears.

By midafternoon, Facebook set an intra-day low of $27.39, before rising to close at $27.72, down $1.88 or 6.4 percent. The fall came despite a new outperform recommendation from Robert W. Baird, with a price target of $37. There were also reports of new collaboration with Apple (Nasdaq: AAPL) on the latter company's next-generation iOS, following comments by Apple CEO Timothy Cook on Tuesday.

Also, Pivotal Research upgraded shares of the Menlo Park, Calif., company from sell to hold despite its plunge from its initial public offering price of $38 set on May 17.

Ford Motor Co. (NYSE:F), the No. 2 U.S. car maker, said it would expand its ads on Facebook. Ford shares fell 44 cents to $10.12.

Two weeks ago, General Motors Corp. (NYSE: GM), the No. 1 auto maker, withdrew its $10 million account on the grounds that Facebook's platform hadn't been an effective advertising platform.

Meanwhile, Facebook acknowledged two glitches late Thursday that temporarily slowed or halted activity on the site, which it claims has 901 million members as of March 31.

Two weeks after the biggest technology IPO in history, as well as the second-largest IPO ever, Facebook has now lost $44.37 billion in market capitalization from its initial $104 billion. CEO Mark Zuckerberg, meanwhile, has been honeymooning while the value of his shares has dwindled to $13.95 billion from $19.13 billion, a paper loss probably unprecedented in value and duration for an individual.

No Facebook official has made a public comment about the share price since the IPO. On Thursday, James Gorman, the CEO of Morgan Stanley (NYSE: MS), which led the IPO, said no nefarious activity had been committed, and there was enormous retail demand from all over the world for Facebook shares. An investigation into initial Nasdaq trading on May 18 by the U.S. Securities and Exchange Commission and other regulators continues.