http://blogs.mediapost.com/spin/?p=1085 "What did we learn? A lot. We learned that most people do not click on ads, and those that do are by no means representative of Web users at large. Ninety-nine percent of Web users do not click on ads on a monthly basis. Of the 1% that do, most only click once a month. Less than two tenths of one percent click more often. That tiny percentage makes up the vast majority of banner ad clicks. Who are these “heavy clickers� They are predominantly female, indexing at a rate almost double the male population. They are older. They are predominantly Midwesterners, with some concentrations in Mid-Atlantic States and in New England. What kinds of content do they like to view when they are on the Web? Not surprisingly, they look at sweepstakes far more than any other kind of content. Yes, these are the same people that tend to open direct mail and love to talk to telemarketers. "
Many people click from site to site. This means that some do click on ads. If you can optimize the ads in a large way so they don't look like ads and look like normal external site links, then you normally receive a higher CTR. That's the problem with Adsense and other ad networks is that you can't optimize them enough because you have to have the "ads by Google" on them.