This analysis of the current state of Internet ads suggests that some of the big ad brokers on the Internet (e.g. Google, among others) may be near an inflection point with respect to ad demand. Lookery, a firm that sells ads on sites like FaceBook and MySpace, just lowered the cost of its ads by 40%, suggesting very soft demand. And Google's AdWord system, according to the article, seems to be propped up financially by Google's practice of setting very high minimum cost per click fees.
I have always maintained that Google is making a lot of money from businesses that are willing to pay modest amounts month after month for a few ads. When times are good, it does not look like a lot of money. But when the cost of other business necessities like fuel and shipping go up, unproductive ads producing few results may be the first to go.