Update on 2/25: FeedBurner has published a post discussing this same issue but providing numbers for their whole userbase, which makes it even more interesting.
Ever since the Reader team announced that we were making public subscriber counts (thanks Justin), bloggers have been excitedly posting about the bumps they're seeing in their subscriber stats. I'm obviously very happy that Reader is getting all this attention, and that we turn out to be quite popular when compared to other feed readers. However, these statistics need a bit of interpretation. Most people post charts of their subscriber counts, like this one for this blog:
For web-based readers where feeds are fetched on behalf of multiple users, the subscriber number is based on what the site reports. To the best of my knowledge, with the exception of My Yahoo!, these number are total subscribers, even if an account is inactive. Unless the site is aggressive about cleaning up inactive accounts, these numbers are only upper bounds on the number of actual readers that you have.
A more interesting number to look at is how many viewers each item gets from each feed reader. FeedBurner provides this as part of their TotalStats package. By embedding a small tracking image in your burned posts and looking at referrers, it's possible to see these item-specific views. Here are how many views and clicks my post from yesterday got in various feed readers:
From this it would appear that Reader has an even bigger lead over Bloglines (though given the biases in this blog's readership, I'm not reading too much into this). There are other factors involved here too. The user bases for feed readers are not identical, if an item appeals more to one population than another, that may skew things. Additionally, some readers (especially homepage-style ones like My Yahoo!, Google Personalized Homepage and Netvibes) don't have to display the item body and allow users to jump straight to the post page. These would show up in the "Clicks" column but not in the "Views" one.
What becomes apparent is that none of these statistics provide a complete picture of your readership, but that when used together they can still give you broad trends and help you tailor your content to your audience.