One theme that's come up recently in several 5by5 podcasts I listen to—especially John Siracusa's Hypercritical—is the idea that advertising-supported models for media content or social networking tend to proliferate because people "don't want to" pay for them or "don't value" the content as much as advertisers value access to the viewers.
Clearly that's part of the story. But I want to emphasize that a lot of the issue is about competition. People do a lot of their "reading stuff on the Web" when they're at their desks and supposed to be working. But you can only get away with so much of this before you get fired. Then you have this other time when you're not at your desk and either need to take care of errands or else could go do something more fun than reading on the Internet. This creates a situation in which the total quantity of potentially amusing Internet content can easily exceed the amount of time you have available to read it. That pushes the price people are willing to pay very low, not because they don't value content but simply because there's so much content out there.