It is an extremely powerful tool; we’ve been busy integrating it into our product for the past few weeks and it is giving us tremendous insights. Happy to share below
Currently we base a cohort on someones yearly-quarter-date-of-first-post (so if you posted your first contribution on 12 March 2015, you’d be in the Q1 2015 cohort), and we show the active-members metric (number of members that have posted in the past 28 days).
This is a typical one (Discourse Meta)
You often see that the first (couple of) cohort remains active; that makes sense since this is the group that includes the founders and the early members. After that you often see that cohorts generally decline.
Here we see a rapidly, steadily growing community; every cohort adds some members that tend to stick around. If your chart looks like this, you’re doing something right*.
This is a typical support forum, you can see that the vast majority of people only sticks around for a single quarter; note that the first two cohorts remain active (they are probably company staff).
I really really love this one, it shows a very seasonal community (gardening focused), but it also shows that they have new members participating nearly regardless of the season. It has a healthy base and a portion of nearly every cohort stick around.
* assuming increased engagement is one of your goals