After months of media anticipation, Facebook launched on the stock market with shares at a starting price of $38, although that is expected to increase by as much as 50 per cent today.
Facebook was valued at $104bn, and Mark Zuckerberg's share at $19.1bn.
The valuation far outstrips longer established companies like Amazon and Disney.
Critics have said $38 share price is too high - Facebook's valuation is four times higher than Google's $25bn when it launched as a public company in 2004. The company expects to have a billion people log on to its site this year, and 400m users log on six days per week.
Mark Zuckerberg is now the 28th richest person in the world, just eight years after founding Facebook at Harvard. The IPO is expected to mint more than a thousand millionaires at the company.
Zuckerberg chose to ring the Nasdaq bell remotely from Facebook headquarters in California, flanked by colleagues who had been enjoying an all night 'Hackathon' at headquarters, writing new code for the site. Facebook will begin trading under the Nasdaq symbol “FB”.
Using the Google model, Zuckerberg created two classes of Facebook stock, so that executives can keep control as Wall Street pressures start to develop. This means that Zuckerberg will have final say on how 56 per cent of Facebook stock votes.
Earlier on, a Nasdaq glitch looked like it would delay the floating indefinitely, but the problem was resolved in time for a 11:30 ET opening.
Co-Founder Eduardo Saverin, whose 4 per cent shares are now worth $4.2bn, has come in for criticism in the US in recent days, as he gave up US citizenship to reside in Singapore – meaning he will not pay capital gains tax on his earnings, as the tax does not exist there. He has claimed this was not the reason for the move.