Although the Alexa rank may not be 100% accurate, as to a lot of professional traffic estimate tools out there, it still serves as a guideline on estimated traffic to a particular website. If that website has a good amount of traffic, it still bumps the Alexa rank higher - thus you can't say that Alexa's ranking is wrong, etc. Although it may not be able to keep track of the traffic 100%, a high traffic site would definitely command a low Alexa rank, and it can still serve as a useful tool in gauging the overall traffic that site is receiving.