From - comscore.com/Insights/Press_Releases/2009/5/comScore_Announced_Media_Metrix_360

"The new approach combines person-level measurement from comScore's proprietary 2 million person global panel with Web site server metrics in order to account for 100 percent of a Web site's audience."

It's possible that Yahoo and Google are providing server metrics via javascript tagging to comScore. That would give them direct access to the traffic data. I believe that Quantcast and maybe Nielsen both offer something similar as well.