My sense is that cross-platform following can mostly be achieved by RSS subscription, although following does include a notification aspect. For example, in both Typepad and Tumblr I can expose the list of those that I am following, and those that are following me. Various platforms need only to notify each other of following and unfollowing, and the identities involved, and then cross-platform following semantics works.
I agree with using RSS subscription as a cross-platform following mechanism but I’d stop there. If you’re gonna use RSS over HTTP then you can use the unique visitors metric as your “followers” count and the referrer information to find out who those followers are.
This, of course, only makes sense if you have a ready to use application that manages all those metrics for you.
Read the original: More On Cross-Platform Tumbling: Following Is Mostly RSS on /Message.