I haven't seen confirmation, but they probably use Alexa given they make it easy to download their top 1 million list: http://s3.amazonaws.com/alexa-static/top

I haven't seen confirmation, but they probably use Alexa given they make it easy to download their top 1 million list: http://s3.amazonaws.com/alexa-static/top-1m.csv.zip http://s3.amazonaws.com/alexa-static/top-1m.csv.zip