Welcome to Tech Bubble 2.0 -- when's the bust?

People want to know: Are we in a tech bubble?

From LinkedIn to Groupon to Facebook, technology companies are taking their social media know-how public -- and they're hitting the markets with dizzying valuations.

This thirst for high tech has left many venture capitalists and analysts worrying that the markets are partying like it's 1999. Investors who were around the last time Wall Street got punch drunk on the Internet remember all too well how that one ended.

"I give this between six months to one year, and that's being optimistic," Vivek Wadhwa, a visiting scholar at U.C. Berkeley's School of Information and an advisor to startups, told TheWrap. "It can't last. It doesn't make sense. It's a mad rush to go public, and they're rushing out as fast as they can, because they know that before the bubble pops they might as well go make their big money."

So far, at least, the markets disagree.

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Facebook's valuation is flirting with $100 billion; Twitter's is kissing $8 billion. Resume-sharing giant LinkedIn hit the New York Stock Exchange last May with a $10 billion market cap, while music sharing hub Pandora made its public debut with a $2 billion value despite the fact it loses money. Waiting in the wings: $1 billion Zynga and $750 million Groupon IPOs.

Unlike the last go round, the businesses that are driving investors wild are coming to market with real track records, not just cool ideas, power-point presentations and get-rich-quick schemes.

Gone, investors hope, are the days of Pets.com, which raised and lost more than $300 million at the height of the last bubble and became a textbook example of irrational exuberance.

This time, many of the companies going public are in the black. Some, such as Facebook and its rumored $1 billion in profits, are true success stories. Others aren't profitable yet. But while the likes of LinkedIn and Groupon aren't making money, they have seen big jumps in revenues, rising from about $78 million in 2008 to roughly $243 million in 2010 at LinkedIn and from $94 million in 2008 to $713 million in 2010 over at Groupon.

"I would not characterize it as a bubble like 2000 or 2001 simply because if you look at the companies that have gone public, they have substantial businesses which are market leaders with revenue growth in the double digits," Eric Hippeau, the former CEO of the Huffington Post and a partner at Lerer Ventures, told TheWrap. "We've been in a desert for IPOs, so it's normal people would flock to Internet companies that are going out now."

Frank Quattrone, who presided over the last tech bubble and helped companies like Netscape and Amazon go public, cautioned at a Fortune tech conference on Wednesday: "I don't think we're close to a bubble if you're talking about companies without value ... But funds are being formed with the specific purpose of investing in these hot things. And unless these companies perform quite well for number of years, some of these valuations might not hold up."

There is clearly money to be made in today's hothouse technology climate. Be warned, however, because in some respects the brand recognition and relative strength of this generation of social networking and gaming companies is preventing investors from looking at whether these heady valuations are sustainable.

"Generally, companies have revenues, which is better and worse," Paul Kedrosky, an investor and contributing editor for Bloomberg, told TheWrap. "It is better in that the companies are more real, but it is worse in that in being more real, it attracts many more people who feel more justified in their beliefs."

In the rapidly shifting digital space, even a proven record of success is no indication that a tech titan can retain its behemoth status for long. The recent sale of MySpace for pennies on the digital dollar shows just how fleeting that kind of popularity can be.