From the Politico:

To get some small read on that online activity, Compete.com, a company founded in 2000 that measures and analyzes Web traffic, studied the habits of hundreds of thousands of online political readers. The nonpartisan, for-profit company uses what's called a "panel" of roughly 2 million Internet users who have agreed to allow their Web behavior to be tracked and analyzed for research purposes. (Other well-regarded companies that measure Web traffic also use a panel-based tracking system, including Nielsen, comScore and Hitwise.)

The study, performed for Politico, is simply a snapshot of online activity; it followed only those readers projected to have visited an official candidate website at least once in September.

Find out the results here.