Regulators are examining whether Morgan Stanley, the investment bank that shepherded Facebook through its stock market flotation last week, selectively informed clients of an analyst's negative report about the company before the shares started trading.
Rick Ketchum, head of the Financial Industry Regulatory Authority, the self-policing body for the securities industry, said the question is "a matter of regulatory concern" for his organisation and the Securities and Exchange Commission.
The top securities regulator for Massachusetts, William Galvin, said he had subpoenaed Morgan Stanley. He said his office is investigating whether Morgan Stanley divulged to only some clients that one of its analysts had cut his revenue estimates for Facebook before the stock hit the market on Friday.
The bank said on Tuesday that it "followed the same procedures for the Facebook offering that it follows for all IPOs" - initial public offerings of stock. It said its procedures complied with regulations.
The questions about the role played by Morgan Stanley, the lead underwriter for the deal, add to the confusion surrounding Facebook's IPO. In the most hotly anticipated stock debut in years, the offering raised 16 billion US dollars (£10.1 billion) for the social networking company, valuing it at 104 billion US dollars (£65.8 billion).
On Tuesday, Robert Greifeld, chief executive of the Nasdaq Stock Market, acknowledged to shareholders of Nasdaq's parent company that "clearly we had mistakes within the Facebook listing".
The stock debut, originally set for 11am EDT on Friday, was delayed for more than half an hour because of technical problems at Nasdaq. Some brokerages were still sorting out the aftermath on Tuesday.
"Unfortunately, our clients continue to feel the effects of this in some cases," said Stephen Austin, a spokesman for Fidelity Investments, one of the country's largest brokerages. Fidelity was still waiting for some Facebook stock orders that it placed on Friday to be executed. Fidelity's systems had performed normally, Mr Austin said.
In the meantime, Facebook stock itself has been a disappointment. It fell 3.03 US dollars (£1.92) on Tuesday to close at 31 US dollars (£19.62) and has now fallen 7 US dollars (£4.43), or more than 18%, from its offering price of 38 US dollars (£24.05).
It managed to add just 23 cents (15p) in its first hours of trading on Friday, then suffered a big decline on Monday.