An echo of 1990s-style dotcom euphoria reverberated around Wall Street as shares in LinkedIn, the business-focused social network, more than doubled on their first day of trading, valuing it at nearly $10bn.

The strong receptionpointed to pent-up demand for the Facebook generation of young internet companies and is expected to be followed by a spate of initial public offers from other companies linked to the online social networking revolution. But it also drew warnings that a new internet bubble might be in the making, with investors rushing to pay prices far higher than a level that was considered extravagant only days before.

LinkedIn's shares were trading at $104, up more than 130 per cent on the $45 IPO price. At that price, it is valued at some $9.8bn, almost triple what it began the week at when bankers put a price tag of $32-$35 on the shares. That leaves it at 40 times last year's sales, on par with where Facebook is valued on private markets.

"This is a very rich valuation which is reminiscent of the year 2000 exuberance," said Michael Yoshikami, chief investment strategist at YCMNET Advisors, a wealthmanagement firm.

"To some extent, investors are buying LinkedIn because they cannot get into Facebook. People are just really desperate to get into social media," he said.

"It's hard to justify, based on [LinkedIn's] current performance," added Ryan Jacob of the Jacob Internet Fund.

The excitement was thick in the air on the floor of the New York Stock Exchange. A crowd gathered at the station of Bank of America Merrill Lynch - the designated market maker which ran the openingauction - in one of the rare moments in the electronic age when the NYSE floor still buzzes with energy.