I have said it before and I'll say it again - Alexa sucks. Here is a new article outlining some of the reasons it is worthless. It's a good read and important to note.
As far as I know the best way to determine a website's popularity would be to combine all of the log file data from all of the DNS servers in the world and create a database that would be searchable and ranked. Since this will never happen, Alexa is seen as the next best thing. Bottom line - it's time to stop looking to Alexa for site popularity.
