blog.pmarca.com:

"...you can't believe any of the Internet measurement companies for any kind of accurate external analysis of Ning usage and traffic -- or, for that matter, usage and traffic of any web site other than perhaps the very largest.

I'm talking about Compete, Quantcast, Alexa, and even Comscore -- none of their data maps in any way to numbers or patterns we see in our own server logs and activity metrics.

This is a well-known problem in the Internet startup world and isn't discussed often enough. It's also a very complicated problem due to a whole hornet's nest of issues, including toolbar adoption, demographic spreads, technology variations, international usage, and domain mapping."

Right on.