vik_c - 2:07 am on Jan 4, 2012 (gmt 0)

A vast number of web sites (probably the majority) do NOT use GA.

That's true but it hasn't stopped Quantcast and Alexa from putting out (estimated) numbers. Considering that Google accounts for a considerable portion of the traffic of most websites, knowing how much traffic they send a website should be enough for them to provide a reasonable estimate of the site's total traffic. This will not apply always but broadly speaking, it's enough to start a ranking system. I also think this will motivate webmasters to start adding GA to their sites. For one of my (quantified) sites Quantcast under reports traffic by half. Their estimates for sites not 'quantitied' are way off the mark. I believe Google will be far more accurate.