I don't doubt that Yahoo has a shit load of traffic still, but unless Alexa/Quantcast/ComScore/Compete/Nielsen or any of these other so-called 'web traffic measurement services' have direct access to Yahoo and Google's traffic data (they don't), I'd take these all with a grain of salt. These services have never been accurate.
http://moz.com/blog/testing-accuracy-visitor-data-alexa-comp...