SAN FRANCISCO – The number of privately held Silicon Valley startups that are worth more than $1 billion shocks even the executives running those companies.

"I thought we were special," said Phil Libin, chief executive of Evernote, an online consumer service for storing clippings, photos and bits of information as he counted his $1 billion-plus peers.

He started Evernote in 2008 on the eve of the recession and built it methodically.

"A lot of us didn't set out to have a big valuation; we're just trying to build something that lasts," Libin said. "There is no safe industry anymore, even here."

An unprecedented number of high-tech startups – easily 25, possibly more than 40 – are valued at $1 billion or more. Many employees are quietly getting rich, or at least building a big cushion against a crash, as they sell shares to outside investors.

Airbnb, Pinterest, SurveyMonkey and Spotify are among the better-known privately held companies that have reached $1 billion. But many more with less familiar names, including Box, Violin Memory and Zscaler, are selling services to other companies.

"A year from now that might be 100," said Jim Goetz, a partner at Sequoia Capital, a venture capital business.

Sequoia counts a dozen such companies in its portfolio. It is part of what Goetz calls "a permanent change" in the way people are building their companies and financiers are pushing up values.

The owners of these companies say the valuations make them giddy but also create unease. Once $1 billion was a milestone; now it is also a millstone. Bigger expectations must be managed, and greater uncertainty looms.

Investors and executives point to a number of reasons for the high valuations. Interest rates are low, which makes it easy for private equity companies to take large stakes in companies. Young tech millionaires and wealthy foreigners like Yuri Milner, the Russian billionaire, have been private investors, too. As each one puts more money into a startup, it escalates the bidding for the others.

Last month the value of Twitter, which began in 2006, hit $9 billion, based on an offer for employee shares by BlackRock, a global investment manager. On the first day Microsoft sold shares to the public in 1986 it was 11 years old and worth just $778 million. That would be only $1.6 billion, adjusted for inflation.

Pinterest, an online scrapbooking and social networking site with no revenue, became worth $1.5 billion in less than three years. Amazon.com went public in 1997 after only three years but had a valuation of just $438 million. And it had almost $16 million in revenue for fiscal 1997.

Silicon Valley entrepreneurs contend that the price spiral is not a sign of another tech bubble. The high prices are reasonable, they say, because innovations like smartphones and cloud computing will remake a technology industry that is already worth hundreds of billions of dollars.

In addition, many of the billion-dollar companies, including MobileIron, Pure Storage, Marketo, DDS and SurveyMonkey (which two weeks ago raised $794 million, to reach a value of $1.35 billion), sell products and services primarily to other businesses.

Most of the chief executives of these firms are also veterans of the Internet bubble of the late '90s and confess to worries that maybe things are not so different this time. Robert Tinker, 43, the chief executive of MobileIron, drives a 1995 Ford Explorer that has logged 265,000 miles.

"If somebody comes to a job interview here in a $100,000 car, I know he's not hungry," he said. "The reality is, I've taken $94 million in investors' money, and we haven't gone public yet. I feel that responsibility every day."

Concern is growing that the billion-plus club is filling up with companies that look alike.

"Everyone is saying, 'I have a cloud technology, I should be valued at 20 times my sales,' " said Jay Chaudhry, the chief executive of Zscaler. "Some are real, but a lot of others are stretched too thin. They'll languish out there."

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