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One industry dominates the '40 under 40' list

Uber CEO Travis Kalanick
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Uber CEO Travis Kalanick

In the technology world, no question comes up more frequently these days than "Are we in a bubble?" And while it's not up to us at CNBC to answer that question, we do sometimes happen upon little nuggetsdata, anecdotes, tweets and the occasional wild partythat can possibly help others make their own assessments. When we do, we'll share them in a little column we're calling: "Bubble watch?"

If you're a businessperson under 40 and you don't work in technology, good luck catching the attention of Fortune's "40 under 40" list.

Almost half (19 to be exact) of this year's list, released on Thursday, comes from the tech industry. The top three in the rankings—Uber's Travis Kalanick, Airbnb's Brian Chesky and Facebook's Mark Zuckerberg—all hail from the Internet.

Also in the top 10 are WhatsApp co-founder Jan Koum, Yahoo CEO Marissa Mayer and Elizabeth Holmes, the founder of med-tech company Theranos.

It's not a huge surprise that tech would have such outsized representation on the list, given that virtually every industry is being transformed by some combination of mobile, cloud computing, automation and an explosion of data.

Also, those are the attributes that Fortune says it's looking for on its leaderboard: "power, influence, leverage, scale, and ideas that disrupt." But it's worth remembering that tech goes in cycles, and the Silicon Valley market has been pointing solidly north for half a decade now.

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Another reminder of that on Thursday came from the National Venture Capital Association, which reported that venture funds raised $23.8 billion in the first three quarters of 2014, more than they brought in all of last year.

For all its efforts in diversity, tech still remains a man's world. Among the 19 executives that made the Fortune list, only four are female.