Facebook shares jumped more than 10% within minutes of making their stock market debut on the Nasdaq exchange.
Shares in the social networking site rose to $42, having been initially priced at $38 each, before falling back to trade flat.
Founder Mark Zuckerberg, 28, who started Facebook while at Harvard University, officially opened trading on the Nasdaq earlier.
He appeared via a video link from a celebration at the firm’s headquarters.
There had been a delay of about half an hour in the start of trading in Facebook shares, in what analysts say reflected the huge demand for the stock.
The $38 initial share price values the eight-year-old social network site at $104bn (£66bn).
Strong demand had led the company to increase both the price and the number of shares available for sale.
Facebook’s owners are releasing just under a fifth of the company’s total shares, about 421 million, which could raise about $18bn.
Facebook’s valuation means the social network site is worth about the same as internet shopping giant Amazon, and more than the value of stalwarts such as Disney.
The initial public offering (IPO) of the shares is the third-largest in US history, after the financial giant Visa and General Motors.
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Facebook employees have been up all night ahead of the event, taking part in a “hackathon” at the company’s headquarters in Menlo Park, California.
It is an event in which programmers work on projects and come up with new ideas.