Paradoxically, one of the major underlying reasons for the decline in CTR is the success of online display advertising and the exponential increase in the number of banners that users are exposed to. MediaMind found that users who are exposed to relatively few ads are more likely to have a high CTR as compared to users who are exposed to a high number of ads. The decline in CTR happens even though in absolute terms, users who are exposed to a high number of ads click more."
Source: MediaMind blog, 16th November 2010
[Note - 0.09% CTR means that you get 9 clicks per 10,000 ad impressions]