Alexa Site Info is a traffic tracking service that attempts to provide an image of the traffic of a website compared to other websites. Alexa also attempts to catalog any back links to a domain. Alexa calculates traffic score from metrics obtained through their browser toolbar. Therefore the traffic stats are really just based on statistical sampling collected from a minority of internet users. Even so it is one of the most accurate traffic indications for a domain obtainable without owning the domain.
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Alexa Site Info 
