Online advertising grew at 25% this year and the advertising industry thinks it would have grown at a much greater rate if there were more accurate ways to measure web traffic. Having been in a number of advertiser meetings myself I can tell you there is much truth to the idea that advertisers are still scratching their heads at the various ways to measure traffic and moreover the variations that exist between these methods.
Here is a detailed Associated Press article on the state of web advertising that is worth a read if you have interest in the topic.