The number of page views will continue to grow exponentially, but ad budgets might grow at 10 percent per year at best. When you marry those two things, CPMs can only go down. Advertising is no longer going to be a high-value product. For publishers, that means you have to figure out how to drive your long-term relationships to get your audience to buy higher-margin projects from you.
via How Gigaom Built a Media Business Around Free Content | Mediashift | PBS.