Bloggers and others are notoriously obcessed with metrics: how many hits, what's my rank etc. As a result there are numerous services which try to analyze sites and blogs etc, to help measure this. Not insignificantly, a site's revenue potential is tied closely to this as well. Seomoz.org has put together a pretty interesting study doing a somehwhat empirical analysis of this question.
"This project's primary objective is to determine the relative levels ofYou should read it. It turns out that they think that Technorati does the best of all of them, altothough tnone of them are especially accurate.
accuracy for external metrics (from sites like Technorati, Alexa, Compete,
etc.) in comparison to actual visitor traffic data provided by analytics
programs." (from Seomoz.org)
"Technorati is doing an admirable job of comparing link data and blogbut then:
popularity, given that it does not claim to measure traffic levels" (from Seomoz.org)
"The sad conclusion is that right now, no publicly available competitive
analysis tool we're aware of provides solid value. Let's hope the next few
years provide better data." ( fromSeomoz.org)