Steve Rubel says something important about measuring the success of your website / blog with traffic as a ruler:

Traffic is not the primary way to measure a blog’s impact. Like PR, the return on investment is more soft, not hard. This needs to be taken into account. For example, if your goal is to promote thought leadership, I would track mentions on Google, media references to the blog, PubSub counts and more.

I think this is a very important point. Who’s reading is more important then how many.