Does Gallup poll every US citizen? These stats are sufficiently random to mirror trends, especially when there's not many other ways of getting this information.
Also, it's one of the only few samplings we have, unfortunately.
"No-one I know - from the savviest sysadmin who only browses in lynx to the most malware-infested bloated-PC user - has ever used Alexa toolbar."
I'm one of "those guys" who everyone comes to in order to get their shitty computer fixed for free. I've seen a lot of "normal users". I do not think that Alexa users are a representative sample based on what I've seen.