There's an Adobe study that shows that Google appears to be cutting impressions (of bad ads?) to boost CTRs and CPC; the same report shows Facebook is handily clobbering Google in CTR / relevance:

* http://www.zdnet.com/article/googles-mobilegeddon-moves-hitt...

Perhaps Google is trying to boost CTR / relevance in order to attract/retain the more valuable advertisers (who would probably otherwise weight Facebook more highly for its better targeting). I really wonder how much of this growth is just Google (and Facebook) extracting more marketing dollars from the biggest spenders.