Concerns over Alexa rank information and the Alexa Toolbar

Alexa ranks sites based on visits from users of its Alexa Toolbar for Internet Explorer and from integrated sidebars in Mozilla and Netscape. In addition to their own statusbar extension, Sparky (released July 2007), there are several third-party extensions for Mozilla Firefox:
SearchStatus shows Google PageRank and Alexa TrafficRank
About This Site Firefox plug-in that shows metadata from Alexa TrafficRank.

There is some controversy over how representative Alexa's user base is of typical Internet behavior. If Alexa's user base is a fair statistical sample of the Internet user population (e.g., a random sample of sufficient size), Alexa's ranking should be quite accurate. In reality, not much is known about the sample and possible sampling biases. Alexa itself notes several examples. A known source of bias is the self-selecting, opt-in nature of Alexa traffic tracking software installation, but the significance of this bias on rankings is not reported.

On April 16, 2008 many users reported dramatic shifts in their Alexa rankings. Alexa confirmed this later in the day with an announcement that they had released the "New Alexa Ranking System" claiming that they now take into account more data sources.

Spyware
The Alexa toolbar is regarded by many vendors as spyware. Symantec classifies the toolbar as Trackware. McAfee Site Advisor rates the Alexa website as yellow with a warning, "In our tests, we found downloads on this site that some people consider adware, spyware or other potentially unwanted programs," and the site has 67 user ratings of "Adware, spyware, or viruses" as of July 2, 2008. The toolbar is detected by McAfee as Adware-Alexa, a "Potentially Unwanted Program." Many other security vendors also detect and remove Alexa software.