This is a good question for Daniel Horiwitz, Elaine Collins, Stephen Phillips, and any other site owner that has a dominant non-U.S. user base.
I know that Alexa and Quantcast and even internal traffic tracking applications like Omniture and Google Analytics typically under report traffic. Obviously, user installed and managed traffic reporting is going to be more accurate than 3rd party reporting.
However, is it a concern by non-U.S. sites that Alexa and Quantcast under-report traffic when compared to sites with mostly U.S. traffic?
Does anyone have any experience with this?