The Pew Research Center's Internet & American Life Project has released Search Engine Use 2012.
Here's an excerpt:
For more than a decade, Pew Internet data has consistently shown that search engine use is one of the most popular online activities, rivaled only by email as an internet pursuit. In January 2002, 52% of all Americans used search engines. In February 2012 that figure grew to 73% of all Americans. On any given day in early 2012, more than half of adults using the internet use a search engine (59%). That is double the 30% of internet users who were using search engines on a typical day in 2004. And people's frequency of using search engines has jumped dramatically.