This are findings of the University of Texas/Chitika Study for US (2006):
But why does someone advertise on Blogs
"Ad revenue in a blog is more sensitive to the rank of the blog than what one would expect in a typical Zipf Law 80/20 curve situation. One reason for this may be the social value of advertising in a blog. If online advertising is like advertising in a mall, advertising in the blogosphere is like advertising in a country club ..."
Forget about reach!
Full Chitika Blog$ Report (PDF)