I think advertising may be the next bubble. I mean, there have been lots of comments online about how large a percentage of ad clicks are fraud, so it's likely most sites shouldn't be making anywhere near as much cash from their ads once that's figured out.

Add the increasing usage of adblockers, fears of privacy regarding tracking and ads plus a stupidly high amount of competition, and I can see ads not being able to fund the sites they need to soon. Complete with various news sites, social networks and platforms simply being unable to pay their bills without finding a new payment model.