Pageviews



This was taken from Alexa's website directly:

Alexa gets its traffic data, including reach, page views and rank information, from a global panel of web users. The panel is used as a statistical sample of Internet usage to extrapolate overall traffic patterns and web usage information. The panel consists of Alexa Toolbar users and other sources web usage information. Details about Alexa’s traffic data can be found on Alexa.com.
 
They start out with a bunch of good guesses and a toolbar, then they get mturk.com experts to add up the numbers, calculate some averages, and create the html make it look almost believable. Pretty cool eh?