Isn't it a bubble, when a lot of startups provide free services that are supported by online ads? Wouldn't a crash in the online ads market have a resonance cascade through all of those startups?
There already was a crash of ad banner rates in 2008, in fact that killed many small publishers. You'd have a real bubble if there was a ton of moneys still being pumped into AdTech, but that hasn't been the case for a year now. And you're now seeing mergers and acquisitions take place. A good example of this on a bigger level would be Verizon buying up AOL for their ad business.
Although something to realize is that there are winners in that business. Most people don't think of Facebook as an ad company, but they really do get mobile advertising and from the looks of things they'll keep growing. And that's not just Facebook but other properties like Instagram which will snag more and more ad dollars.
That isn't quite a bubble, in fact, what you're describing happened to broadcast television advertising and online ads. If people shifted their attention from online to something else, advertising (and thus startups) would follow.
Although something to realize is that there are winners in that business. Most people don't think of Facebook as an ad company, but they really do get mobile advertising and from the looks of things they'll keep growing. And that's not just Facebook but other properties like Instagram which will snag more and more ad dollars.