Ask HN:Any RSS analytics tool for # of new subscribers and unsubscribers?

I'm trying to calculate retention rate / churn rate for my RSS feed. I'm looking for a service similar to http://tweetake.com/ and http://www.tweeteffect.com/ for twitter. Unfortunately there is no way to identify RSS readers (or maybe there is?), so I don't believe a custom colution is viable (I'm talking as a non-developer, forgive me if I'm wrong). Counting the subscribers one week, and seeing the difference the next week, is not what I'm looking for. In that case, only the net gain or loss is calculated. Any other creative ways are welcome. Thanks, talbina