And how do they actually measure those numbers without (i) guessing; (ii) having some JS on the page, or (iii) using Voodoo Black Magic.
I think it's ridiculous to believe ANY company that claims that they can measure traffic of a site accurately without the DIRECT INVOLVEMENT of the site's owners. How are they doing it? Through the ether? Come on! Data, people. Data. Google's figured that out, maybe other companies should too.