Facebook shares will start trading in New York later in one of the most high-profile share sales of recent years.
Founder Mark Zuckerberg officially opened the day's trading on the Nasdaq exchange, although trading in Facebook shares will not start until later.
He appeared via video link from a celebration at the social network's headquarters in California.
The shares are priced at $38 each, valuing the eight-year-old social network site at $104bn (£66bn).
Strong demand has led Facebook to increase both the price and the number of shares available for sale.
The Nasdaq exchange, which is where technology giants such as Google and Apple are traded, opened at 0930 local time (1430 BST), with buying and selling of Facebook shares set to begin at 1100 local time.
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.
Facebook's owners are releasing just under a fifth of the company's total shares, about 421 million, which could raise about $18bn.
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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.
Facebook's profits are tiny in relation to its size - it makes about $5 a year for each of its 900 million users - and its plans to increase profitability are unclear.
David Kirkpatrick, author of The Facebook Effect, says there is an army of potential stock holders among its users who are likely to push the share price higher.
"People want to own the [Facebook] stock because they love it so much. I find people all the time who are just devoted to Facebook," he said.
The site is largely used for social updates, and although Facebook has said its use on mobile devices are the key to new profits, analysts question how much room there is for advertising on such platforms.
Car giant General Motors added to those doubts by saying on Tuesday that it would no longer pay to advertise on the site.
The feverish anticipation for this market debut did not extend to all investors.
Oliver Pursche, president of Gary Goldberg Financial Services, told the BBC ahead of the flotation: "We're telling our investors to hold off.
"Number one, we don't know what the guts and the balance sheet of the company looks like yet so that's a big red flag for us. We want to understand the business before we tell people to invest."
The new shareholders will not have much say in how the business is run.
The shares on offer are "A" shares, which carry one vote per share, as is normal, but the current owners' shares are "B" shares, which carry 10 votes each.
They will control more than 96% of the votes after the flotation, with founder Mark Zuckerberg holding just under 56% of the voting power of the company.
Mr Zuckerberg, who owns about 25% of the company, stands to gain the most from taking Facebook public. Fellow founders Dustin Moskovitz and Eduardo Saverin will also become paper-billionaires overnight, as will Napster founder and former employee Sean Parker.
US venture capital firm Accel Partners and Russian internet investment group Digital Sky Technologies also hold significant stakes in Facebook, while software giant Microsoft and U2 frontman Bono also stand to make a huge profit on their investment in the company.
Source BBC News