SAN FRANCISCO (Reuters) - The fallout from Facebook Inc's messy initial public offering widened on Wednesday as shareholders sued the social network and its bankers while a trading firm revealed a massive loss on the shares and threatened to seek "remedies." The Nasdaq stock exchange also came under further pressure as a source close to the situation told Reuters that NYSE Euronext had opened discussions with Facebook about a potential stock listing there. Nasdaq also faces litigation from angry investors. Facebook's listing, envisioned as a crowning moment for an eight-year-old company that has become a business and cultural phenomenon, has instead turned into a legal and public relations fiasco for the company and its lead underwriter, Morgan Stanley. Serious trading glitches interfered with the stock's opening on Friday, and subsequent revelations by Reuters that analysts had quietly reduced their revenue forecasts prior to the IPO have led to accusations of selective disclosure of material information. The shares closed at $32 on Wednesday, 15 percent below the IPO price. A lawsuit filed on Wednesday seeking class-action status alleged that defendants -- including Facebook, its Chief Executive Mark Zuckerberg, Morgan Stanley, Goldman Sachs Group Inc and JPMorgan Chase & Co -- concealed "a severe and pronounced reduction" in revenue growth forecasts resulting from greater use of Facebook's app or website through mobile devices. It also accused Facebook of telling its bank underwriters to "materially lower" their forecasts for the company. The lawsuit said the underwriters disclosed the lowered forecasts to "preferred" investors only. "The main underwriters in the middle of the roadshow reduced their estimates and didn't tell everyone," said Samuel Rudman, a partner at Robbins Geller Rudman & Dowd, which brought the lawsuit. The firm is among the leading securities class actions firms in the country. "I don't think any investor in Facebook wouldn't have wanted to know that information." Andrew Noyes, a Facebook spokesman, said: "We believe the lawsuit is without merit and will defend ourselves vigorously." Morgan Stanley had no comment. It said on Tuesday that Facebook IPO procedures complied with all applicable regulations and were the same as in any initial offering. Also on Wednesday, Knight Capital Group Inc said its second-quarter results will be hurt by losses related to numerous issues during the listing. The firm, which provides electronic trading services to brokers and retail clients, foresees a pre-tax loss of $30 million to $35 million related to the IPO. The company has submitted claims for financial compensation from Nasdaq OMX and is considering all legal remedies available, Knight Capital said in a regulatory filing. Knight Capital's announcement may be a sign of things to come as other traders and investors tally up losses from the trading problems. Nasdaq OMX was also sued on Tuesday by an investor who claimed the exchange operator was negligent in handling orders for Facebook shares. Morgan Stanley said it is reviewing Facebook trades and would adjust prices for some retail customers who overpaid. A source familiar with the situation told Reuters that NYSE Euronext had opened discussions with Facebook about a potential stock listing there, and that the social networking giant was considering its options. The largest U.S. exchange later denied it was discussing a full listing transfer with the company, which became the first U.S. company to debut with a market value of over $100 billion. IPO INVESTIGATIONS Wednesday's lawsuit, one of several that have been filed around the country, was brought in New York on behalf of Dennis Palkon and Brian Roffe, who said they respectively bought 1,800 and 200 Facebook shares at the IPO price, and Jacob Salzmann, who said he paid more than $123,000 on May 18 for 2,961 shares at an average $41.77 each. Research analysts at several underwriters lowered their forecasts for Facebook after the Menlo Park, California-based company in a May 9 prospectus cautioned investors about the possible impact of users shifting to mobile platforms. Facebook currently makes little revenue from mobile ads. Citing people with direct knowledge of the matter, Reuters reported this week that, during its IPO road show, Facebook advised analysts for its underwriters to reduce their profit and revenue forecasts. The lawsuit named underwriters Morgan Stanley, Goldman Sachs, JPMorgan and Bank of America Corp as having cut their forecasts after the May 9 prospectus was filed, but that these cuts were not publicly revealed before the IPO. [ID:nL4E8GM8SK][ID:nL1E8GN0FT] The plaintiff-shareholders called the disclosures of Facebook's business risks inadequate, saying that analysts knew more about these risks and cut their business outlooks accordingly -- for the benefit of only some investors, not all. "If Facebook told analysts to materially lower their forecasts, it should have told the entire market," said Antony Page, a professor at the Indiana University Robert H. McKinney School of Law. "We need to know what exactly was said to the analysts, and determine how different Facebook's public story was from its private story." Regulators including the U.S. Securities and Exchange Commission, the Financial Industry Regulatory Authority, and Massachusetts Secretary of the Commonwealth William Galvin are now looking into how the IPO was handled. The U.S. Senate Banking Committee is also reviewing the matter. "If Facebook faced a known and particularly salient risk, boilerplate language would be insufficient," said Elizabeth Nowicki, an associate professor at Tulane University Law School and a former SEC lawyer. Bank of America and Barclays Plc are also defendants in the New York case, as are Facebook Chief Financial Officer David Ebersman and several Facebook directors. Bank of America spokesman Bill Halldin, Barclays spokesman Mark Lane and Goldman spokesman Michael DuVally declined to comment. JPMorgan did not respond to requests for a comment. The case is Brian Roffe Profit Sharing Plan et al v. Facebook Inc et al, U.S. District Court, Southern District of New York, No. 12-04081.