Robert Scoble has an interesting post about the unreliability of Web stats.

Almost every entrepreneur I talk to lately whines privately about the stats they see on places like Compete.com, Comscore, and Alexa. Today Tom Conrad of Pandora told me that they are extremely low. He says his service requires registration, so he has very accurate stats of who’s signed into Pandora and he can’t figure out why the stats services are so far off of the real stats.

Lots more in the post. It’s also attracted some very thoughtful comments.