LinkedIn went public Thursday and, as expected, soared in its debut. But the social network for professionals outdid even the wildest expectations in what has become a red-hot IPO environment, ending the session up nearly 110% above its opening price of $45.

LinkedIn never traded below $80, and closed out its first day as a public company at $94.25 — providing a windfall for the Wall Street insiders who participated in the offering, as well as the private investors who bought shares on secondary markets. LinkedIn raised over $100 million from Sequoia Capital, Greylock Partners and Bessemer Venture Capital Ventures. Morgan Stanley, Bank of America, Merrill Lynch, and J.P. Morgan were the lead book-runners for the IPO. UBS and boutique tech-media firm Allen & Co. assisted.

“The guys who bought on the private markets going into this IPO got a fantastic deal,” said Espen Robak, president of Pluris Valuation Advisors, which helps hedge funds and other institutional investors price startups.

But LinkedIn’s eye-popping valuation — $10 billion, as of today — also fueled fears that the boom-mentality that abruptly ended the first internet IPO revolution is happening again.

“I do not understand how this valuation can be supported,” said Tim Keating, president of Denver-based Keating Capital, a boutique $53-million late-stage venture capital fund that invests in pre-IPO companies, but did not invest in LinkedIn. “I think one has to ask oneself if LinkedIn should be trading at many orders of magnitude above their projected earnings.”

LinkedIn, which has more than 100 million members, earned $15.4 million on $243 million in revenue last year.

The optimists saw LinkedIn’s debut as confirmation that after years of relative inactivity, the tech IPO market is starting to come back to life for companies with real business prospects.

“There was a period there of very little activity during the recession with a lot of money on the sidelines, and I think we’re playing catch-up now,” said Rick Marini, founder and CEO of BranchOut, a Facebook-based social recruiting service that has raised $24 million in VC funding. “Venture capitalists are putting money back to work, and companies that have a sustainable business model are being rewarded.”

New tech bubble or not, it’s clear there is massive demand in the public markets to own a piece of these high flying internet names. Web auto rental company ZipCar raised $174 million and watched its stock price close up 60 percent in its first day of trading last month. Demand Media, which went public in January, enjoyed a 33 percent first-day pop. And Chinese internet social networking giant Renren — referred to as “the Facebook of China” — went public early in May, soaring 40 percent higher in its first day of trading.

No doubt a slew of IPO prospects, led by Facebook, which is valued at more than $50 billion and is expected to IPO next year, watched the proceedings with satisfaction. “The secondary market for Facebook, Twitter, and Zynga got a real shot in the arm today,” Robak noted dryly.?