Patrick Ruffini has written a much commented-upon analysis suggesting that the number of visitors to the Daily Kos website may be overstated by 60% or thereabouts. Dean Barnett comments on the political implications.
While both of these posts talk about the political impact of the story, I can't imagine trying to explain Ruffini's methodology to an ad buyer who wants to know how he can be sure there are as many visitors to the site as claimed.
No one in the blogsphere is helped when people advertise on a highly (self) promoted site based on inflated traffic numbers. The reason is that they are likely to be under-whelmed by the number of responses. It seems we have another reason to call him Markos "screw 'em" Moulitsas'.