The ongoing charley horse on the booming online advertising marketplace, in particular with regard to community driven and blog sites, is the lack of any reliable third party traffic estimates. Alexa is a joke, but everyone uses it. Comscore can’t measure these sites well, as Fred points out.
I’ve been at both Google and Yahoo in the past ten days, and privately, folks there both told me that they had the data and the wherewithal to blow Alexa out of the water. They have relationships with tens of millions of customers, and access to billions of clickstreams. Why not create, if only for the PR rub off, a solid estimator of web traffic? Lord knows we need it.