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?
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.