People all over the world are reporting adsense/adwords CTR on content network going down. It is inevetable, because content network has grown at a much faster rate than online population. You see same ads on different pages. Who would be interested in clicking them again and again! Hence the fall in CTR. And, this is the reason all of us have seen Google displaying shifting to Optimized Ad serving. I am against opting for optimized ad serving, because they deliver high CTR ad most often, resulting into fall in the CTR of that ad itself! Also, the chosen ad is by CTR and not by conversion, so it may result in drop in conversions too. This is not the solution either for Google or the advertisers. More important is why people don't click on these ads. Reason is they see same ad at all places they visit. Suitable solution is create a ratio between impresion and no. of ads. Make ads for those 1000 impression, and you would find number of clicks increasing. This situation is beneficial for both Advertiser and Publishers. I am always of the opinion that content network should have large number of text ads. More ads you make, more focused group they get delivered and more exact your message becomes.
P.S. -----> Google Now offers Impression Cap for Display Network ads, which prevents overexposure.