According to this article from 2009 reporting about some consultant study that divided people into "heavy clickers, moderate clickers, light clickers and non-clickers" of ads, between 2007 and 2009, the non-clickers grew from "68 to 84% percent of the entire Internet population."
http://econsultancy.com/us/blog/4720-16-of-users-click-on-ba...
The study also claimed that the more savvy people became about the internet, the less likely they clicked on ads. The population as a whole is becoming more savvy about the internet.