I take alot of this with a grain-of-salt. The numbers are pretty suspect. I look at Twitter and realize they have a ton of traffic that is in the api by clients. Those numbers don't show up in web statistics. There are dozens of ways that numbers flowing in and out of Google could be adjusted from month-to-month and feature-to-feature. It could also mean Google is getting better and people don't have to spend as much time searching as they used too. What used to take 10-15 searches, now only takes 5.
I would like to see a look at "uniques" on a graph. "searches" is like counting page views. Sure, we do 500k views here a day, but only 150k uniques. We can do alot to manipulate that page view, but we can do little to influence that 'unique' stat.