Alexa has a unique ranking system where the measure the ranking of a site based on their toolbar users visiting certain sites, Alexa's ranking criteria can give you an idea how good a site is and how well they get traffic from various sources,

But it is not a definete measure since there are not many Alexa toolbar users, Only Alexa toolbar users are taken into consideration for ALexa site rankings and it is not a good measure,

But Alexa will give an idea of the quality of the site, If there is a hike in traffic to a particular site as reported in Alexa then we can judge definetely there is some increase in popularity for that particular site,

SEO Blog Team,

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