...Finding The Authoritative Search BlogsSource 1: Alexa
Over the years, there've been many critics about the Alexa ranking, and there are plenty ways to game its ranking. There is even one search engine that screens Alexa top ranked websites. Stil, Alexa Rank became a status symbol.
In any case, Alexa is probably the most reliable traffic measurement tool today. But, as Alexa states, only the top 100,000 results should be regarded as reliable. (I.e. if one website is ranked higher then other website and both are ranked less than 100K , it's probably has more traffic than the other).