Google data: clicks versus cost per click
The (further) fall in Google's stock today seems to have been triggered by a comScore report saying that growth in the number of sponsored clicks has decelerated a lot. See the post
at Silicon Alley Insider for details.
However, on the other hand, consider recent trends in the average cost per click according to EfficientFrontier
Google's average CPC certainly seems to be trending higher, so this could be a significant counterbalance to a drop in the number of clicks. I.e. if YoY sponsored click growth is indeed flat, this data would still put revenues up 20% or so, maybe more if you maintain the slope of the average CPC. Whether such revenue growth matches expectations, I don't know (I haven't looked it up and also there are other factors that play into that).
It may be a deliberate attempt on Google's part to reduce low value clicks (some of which they call "accidental"), with the idea that they can get the average CPC up.