Active Users of a feed is a very hard number to try and estimate.
A lot of sites will flaunt RSS numbers, but from casual observation, it seems people rarely unsubscribe from feeds.
Sometimes webmasters and bloggers get a clue when a feed is moved and you can see old subscription numbers vs new subscriptions.
These, and any other blogs shutting down over at Weblogs, Inc. have an opportunity to test out their active RSS base. They can keep the feed urls going and watch the subscription numbers, and how quickly they decrease. Sure, everyone doesn’t unsubscribe right away, but over the course of a few months you should be able to get a good read on things.
I know a lot of people don’t want to know these numbers as they feel it can hurt promotion, but for accuracy’s sake, I think it’d be a good experiment.
Just a thought.