The assigning of a particular Alexa rank to a website or a blog is based on the number of surfers(visitors) to your website. The surfers to the website must have the Alexa toolbar installed on their browsers otherwise they will not be counted. The Alexa rank counts the number of visitors and page views for all the websites on a daily basis.
This specific visitor counting also limit the actual measurement of popularity of a particular website or blog. Visitors not having installed Alexa toolbar will not add to the ranking and therefore the overall ranking results will not reflect a true picture. » Read more: Alexa Ranking:It Adds to Your Blog Value