While studying basic statistics of various communities for a chapter in my book, I made a discovery: healthy and active communities may demonstrate very different metric results. So my conclusion so far is that you should not rely on basic activity stats to measure healthiness of an online community.
I calculated activity per active users in last month:
Topics created per user vary from 0.07 to 0.55 – this makes a huge difference, i.e. 7 new topics vs 55 new topics per 100 active users per month.
Messages created per user vary from 1 to 30, i.e. 0.1k vs 3k new messages per 100 active users per month.
Communities examined are those in different industries, in 4 different languages (English, Spanish, Russian, Ukrainian), and spanning from 1k to 1.5M registered users.
Also, I noticed that the percentage of registered users which get active in last month vary as well: from 0.3% to ~50%.
Sharing this in case it can be useful to someone.
P.S. Of course the Discourse team has a greater insight based on the anonymous statistics they collect from Discourse instances. Yet it looks like analysing 10-20 random communities already does say that activity metrics are nowhere near being signs of community healthiness.