There are a number of metrics that could be considered important for tracking retention on a forum like Reddit. Some possible metrics to consider include:
Engagement can be indicated by new posts, comments, upvotes, sharing & moderation.
Why do you think users come to forums like Reddit?
Users come to forums like Reddit for a variety of reasons, including checking answers to questions, posting their own questions, moderating groups, voicing their opinions, and checking out different subReddits. The more they engage with the forum in these ways, the more the community grows. This can be seen as a sign of a healthy and active community, and it can help to attract and retain more users on the platform.