If you have Google AdSense ads in your Website then depending on the category of your site, you may have noticed that CTR (click through rate) dropped for a while - quite a bit, and then seems to have recovered. Less people may be clicking ads in a recession - they aren't buying. And Google may have less ads in stock for that category in each location, so they were putting up multiple copies of the same advertisement or public service ads. Of course, multiple copies of the same ad are not as effective as different advertisements that attract different clientele.
Google seems to have fixed that a bit by being a bit more broad minded about what ads they will match to what keywords, or in some other way, because CTR has improved slightly. At the same time, they are trying to make up for low CTR in Webpage ads apparently by generating more ads on search pages. Search pages seem to have a much higher CTR than website pages. Google may be trying to improve their numbers for the next quarter earnings report. In any case, Google is number 1 with about 80% share of advertising revenue. (See Google's Ads Per Keyword Output In High Gear)