That doesn't mean that the companies aren't overvalued. I think the David McClure quote puts it decently:
“I’m not saying Quora, Foursquare, Square aren’t eventually worth a lot of money, but the price to pay to get into those games is kind of amazing — $50 to $80 million?” said Dave McClure, founding partner of 500 Startups, a technology incubator in Silicon Valley. “These companies are in big markets with proven founders, so maybe not absolutely crazy but certainly eyebrow-raising.”
"The chief evidence, according to industry experts and analysts, is the way venture capitalists and established companies are clamoring to give money to young companies, including those with only a shred of an idea."
This just seems inaccurate. All of the companies mentioned in the article (and most of the companies that seem to get funding) at least have a functioning product or service. You still have to actually build something.
Take Yammer - it's not difficult to build. People made identi.ca in their spare time, for free. If Yammer turns out to have a real market, competitors in the intranet space can easily add a similar product to their line. How is a $40m VC investment justified? My guess is that the investors are betting on selling Yammer to a bigger company. This is what's known as "find a rich sucker" (or "AOL") business model, and is typical of a bubble.
To determine if it's a bubble, you don't look at the companies or products. You look at the investors. If they are loose with their money, that's when you know.
A minor factual correction: identi.ca is a product of StatusNet, Inc., which has at least three employees at the moment. Evan did not build it "in his spare time". However, it's true that you could take the StatusNet code and use it to build a product like Yammer "in your spare time" if you were a "competitor in the intranet space".
While I'd agree it's incorrect to call Tumblr a niche site (it's demographic is too broad to be niche), I disagree with your logic. Why can't a niche site be in the top 100? If a niche is big enough, why not? Facebook at one time was in the college niche.
Last I checked, Tumblr was on the 100 most visited websites list. That's pretty good for a "niche" blogging site.
This story is pure sensationalism, most of the companies listed have proven business models or at least substantial traction.