I add and delete RSS and Atom feeds from my news aggregators every week. I publish my OPML file on occasion and services such as Dave Winer’s Share Your OPML index the file. Anyone could run a diff of the OPML and modified date and track my subscribes and unsubscribes. What’s lost in the data is why I unsubscribed. I think this unsubscribe data would be valuable to publishers, especially as syndication becomes more corporate. Maybe someone unsubscribes to my feed because they do not like my posts about soccer. I could create a feed that includes or excludes certain categories, or I might already have that capability and the user is not aware of the variety of feeds. A user might dislike a feed with only titles and unsubscribe from the feed. All ideas that could motivate a publisher to change their offerings. I imagine a business (Bloglines, Feedster) could charge owners of a feed for information regarding why a user unsubscribed. User retention spending already exists in the publishing world, and the service would be an easy sell to companies tracking online subscriptions. The first step is centralized sharing of subscription lists. Bloglines and Feedster already allow OPML imports. The second step is allowing tool vendors to pass a change to your servers using XML-RPC or other formats. An aggregator user could choose to share his or her complete list as well as the deletes if they opt-in. Of course the company would offer consulting services to help feed publishers create best practices and grow their subscriptions. Such a service would be the Nielsen ratings of the online syndication world.