“There’s definitely a lot of betting going on, and it’s not rational,” said Tim O’Reilly, a technology conference promoter and book publisher.

Mr. O’Reilly is credited with coining the phrase “Web 2.0,” which refers to a new generation of Web sites that encourage users to contribute material. His Web 2.0 conference, which begins Wednesday in San Francisco, has become a nexus for the optimism around the latest set of society-changing online tools. But that has not stopped Mr. O’Reilly from worrying that the industry is minting too many copycat companies, half-baked business plans and overpriced buyouts.

When the bubble inevitably pops, he said, “there are going to be a lot of people out of work again.”

Putting a value on start-ups has always been a mix of science and speculation. But as in the first dot-com boom and the recent surge in housing, seasoned financial professionals are seeming to indulge in some strange instinct to turn away from the science and lean instead on the speculation.

This time around, people indulging in that optimistic thinking are not mom-and-pop investors or day traders but venture capitalists whose coffers are overflowing with money from university endowments and hedge funds. Many of those financial professionals say that this time, everything is different.

More than 1.3 billion people around the world use the Internet, many with speedy broadband connections and a willingness to immerse themselves in digital culture. The flood of advertising dollars to the Web has become an indomitable trend and a proven way for these start-ups to make money, while the revenue models of the dot-coms of yesteryear were often little more than sleight of hand.

“The environmental factors are much different than they were eight years ago,” said Roelof Botha, a partner at Sequoia Capital and an early backer of YouTube. “The cost of doing business has declined dramatically, and traditional media companies have also woken up to the opportunities of the Web.