Found this about Alexa while looking at different VOIP clients today:

Alexa does not calculate the actual number of users visiting Web sites... what we do instead is release raw data about how many people in a sample population (Alexa Toolbar users, among others) visit a site. The data is normalized to a sample size of 1 million users so that the reach doesn't fluctuate as our sample base grows or shrinks.

Look at how Skype is climbing the chart with close to zero dollar marketing spend whereas Vonage spends millions and remains flat. Viral marketing is simply amazing.

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