Gian Fulconi from ComScore has taken note of the fact that paid search traffic is witnessing a decrease of late. Fulconi says that even though search queries are up 68% over the last couple of years, the growth rate of the number of paid clicks is at just 18%. His hypothesis is that this is because people are using longer search terms, rendering the likelihood of an advertiser bidding to have his or her search results included in the results page less, due to paid advertising strategies that limit ad coverage.
John Battelle puts it very succinctly:

In short, our queries are [...]