After an attention-grabbing half-hour delay to start its initial public offering, Facebook shares opened at $38, surged as much as 11 percent during the day, but ultimately finished just above unchanged after hitting an intraday high of $45.

Late in the trading day the stock threatened to hit negative numbers, vacillating around unchanged as underwriters put up a vigorous fight to defend the breakeven point.

The lackluster performance was both a testament to good pricing and a bit of an embarrassment for a company that was supposed to wow Wall Street on its opening day.

And after the market close the SEC said it would be looking into apparent hiccups in the trading execution of Facebook shares on Nasdaq.

"I thought it was going to open at $48 and trade up around there," said Michael Cohn, chief market strategist at Atlantis Asset Management in New York.