The biggest niche population of Alexa toolbar users is probably internet marketers.
Also incredibly ironic. These people are just seeing what they want to see and creating buzz within their own community rather than measuring something useful.
Assuming "calculating the bias" is a well posed question (and I'm not convinced it is) then measuring that bias is a noble goal for alexa engineers. Surely this is a multi-phd-thesis kind of problem though :)
Huge error bars on a near zero estimate don't mean a whole lot except that the experiment producing the data is flawed.