That solution doesn't address my question. Caching @reply subscribers only addresses the 3% of users who have opted-in to receiving all @replies. For the remaining 97% whether they receive @replies from someone they are following or not is a function of which user the reply is directed to and whether that user is also their friend. You can't cache that, at best you can optimize how you calculate who should receive replies.

In fact, what you've pointed out is that building an implementation to address the 3% case is straightforward which was the point of my post.

PS: My blog does have a commenting feature. In fact there are 3 comments in response to the post.