I think two things are happening: 1. Google's natural results are not as useful as they were before (too much spam!), so more people are using other SEs, which naturally leads to fewer paid ad clicks. 2. Also, for sites in the AdSense network, the Websites' visitors have become attuned to the fact that AdSense ads are just that - ads. So, they are clicking on them less, and instead looking for real content. Oh, I am so glad I quit using AdSense.
It's because of the changes Google is losing it's revenue and we also... A Advertiser will always look at the no of visitors from clicks rather than quality of visitors ,because if 100 visitors are going to visit your site then 100 will know about your site and they may tell it to others (about 100 people)who may be probable future visitors who may also be buy something on the site But if your are going to get 10 quality visitors then you may tell it to less no of people(about 10 people) which in turn brings in less no of future visitors
It doesn't concern me too much. Everything has a cap. Even Google... I don't believe, however, that Google's search results are tainted with too much spam causing people to go use other search engines. nah!