Total number of posts within a feed that the user interacted with (could be something like click-through rate)
The order in which the users interacted with a post on a feed (since we technically list posts in descending order by probability of engagement, then the first posts in a feed should have the highest probability of engagement): https://www.evidentlyai.com/ranking-metrics/ndcg-metric
A weighted sum of the total engagements (e.g., likes, comments, etc.) for a given feed
(For the intervention condition) the "constructiveness" of the comments that the users write.
Some possibilities: