Subscribe to comments on external sites

[akirk]

The Friends plugin could create a user for the external site (maybe with a role Comment Subscription) and follow a particular comment feed for cases when you want to follow a certain post.

For WordPress sites a comment feed exists for each post out of the box, usually just nobody makes anything of it. The feed system of the friends plugin could subscribe to those posts and poll them first hourly like the other feeds, then stop checking after a while.