As Facebook shares fell another 9% on Tuesday, regulators said they may review the disclosure process to see if some investors got favourable access.
The SEC and regulatory body FINRA both said they would look into the matter.
Morgan Stanley, lead underwriter on the flotation, said it was "in compliance with all applicable regulations".
Reuters and the Wall Street Journal reported that Facebook's advisers may have revised their financial forecasts for the social networking company, but that only selected investors were told.
Mary Schapiro, chairman of the Securities and Exchange Commission, said "there are issues that we need to look at".
Richard Ketchum, chief executive of the Financial Industry Regulatory Authority (FINRA), said there were "matters of regulatory concern".
But in a statement, Morgan Stanley spokesman, Pen Pendleton, said the bank had "followed the same procedures for the Facebook offering that it follows for all initial public offerings".
Also on Tuesday, a private investor issued a writ against the Nasdaq stock exchange over technical problems on Friday that disrupted Facebook's first trading day.
Phillip Goldberg, a Maryland resident, is seeking class-action status on behalf of all investors who say they lost money because of the technical problems.
Facebook's market debut was delayed by about half an hour, and orders to buy or sell shares were further disrupted. Mr Goldberg has filed a writ in the southern district court of New York.
Nasdaq chief executive Robert Greifeld has said that "clearly we had mistakes in the Facebook listing".
It has all taken the shine off one of the most anticipated flotations in history.
Facebook shares, launched at $38 each on Friday, fell 11% on Monday and a further 9% on Tuesday. The shares are now worth $31 each.
It is a remarkable turnaround from recent weeks, when the social network's share sale was over-subscribed and the eight-year-old firm was valued at $104 billion.
Much of the blame for the fall in share price is being pinned on lead underwriters Morgan Stanley and the Nasdaq exchange itself, with many commentators saying that the sale was over-priced.
"There must have been some sober second thoughts about this," said Brian Wieser, an analyst at Pivotal Research.
Mr Wieser thinks a fair price for such a young company without a proven business model is about $30.
But some analysts say the fall is of little consequence in the early days.
"Whether [Facebook] is worth $95 billion or $100 billion, it's immaterial," Jeremy Liew of investment company Lightspeed Venture Partners told Reuters.
The social networking site has transformed the way in which hundreds of millions of people around the world communicate. It is also transforming the way companies advertise to existing and potential customers.
But Facebook's 900 million users helped the company generate just $1billion in profit last year, and there are concerns about its ability to grow profits in the future.
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