Alexa ranking is good only for a vague idea of the popularity or the traffic of a site ... unless it is in the top 100,000 , in which case you can get a vague (again) picture of the reach/rank/page views during the last period ...
the results are vague because Alexa only tracks the hits from users that have certain Alexa files (in their computer's registry) . When you install the Internet Explorer it contains some files of Alexa (some people refer to it as a type of "spyware") . If you don't uninstall it it sends data to Alexa and that is how they compile their lists . You can easily uninstall the files with most anti-spywares (eg.
Ad-Aware -freeware-) . Therefore , more seasoned surfers (who usually scan their systems regularly and have probably uninstalled the Alexa files) usually don't send any data to Alexa . It does not need the Alexa toolbar for them to track hits , those files in the IE are enough .
also the results are vague because their "reach" and "page views" results are "per million users" and therefore they do not represent a specific number (absolute number) but a rate (relative number) , since you don't exactly know how many millions are the internet users .
use
this link to get an idea of the total number of internet users
(the "per million" in Alexa , of course does not correspond to this number , but rather to "per million users with Alexa files")
and this
interesting link for real-time world statistics (population , etc)