Make activity pages build a community

As a user I would like to be able to see what the rest of my community is doing. I would like to see if a user in the group I'm in has made a new annotation, if someone has replied to my annotation, if someone has annotated the same article I did, etc. Activity pages do show this information but it's not organized in a way that facilitates this kind of interaction. Instead of showing articles based on a time stamp they should be more like a twitter feed, focusing on the annotation that was made and how it is relevant to the person viewing the feed. Notifications through email were mentioned many times, but that's a solution to a problem and a hacky potentially noisy solution at that. I think activity pages are a much better medium for presenting and facilitating this kind of community building interaction.
Highly has a similar concept using a slack channel as a way of notifying the user.

No files, tests, or entry points are named. Start by reviewing the existing activity pages and how they currently organize articles by timestamp; the issue is done only when the desired community-focused feed behavior and interaction scope have been agreed and implemented.

Written by the indexing model from the issue text.