If you are a blogger then you probably keep an eye out on the 'circulation' of your blog. It's a black art full of subjetivity and highly detailed technical tidbits that have to be understood and then interpretted before you really know what the data means. Still if you are interested, check out this post from FeedBurner Weblog (Burning Questions):
"[snip...]While it's always encouraging to see feed subscription numbers go up (Google's recent inclusion of subscription data for Google Reader and Google Homepage increased the number of feed subscriptions FeedBurner can report to publishers by 53% alone), these subscription numbers in a vacuum only tell part of the story. Equally important is the extent to which people are reading feeds and interacting with them. In today's post we'll take a look at one aspect of the feed aggregator market - the web-based aggregators - and draw some conclusions about how audiences are consuming content in new and different ways. [snip...]"