A thing I have been measuring, and a genuine question at the end of it.
I needed to pick which developer communities to spend time in, and I did not want to judge by how busy the front page looked. Discourse publishes the answer without an API key: GET /about.json returns posts_30_days, topics_30_days and user counts on any Discourse forum. Sixteen forums, one request each.
The spread was much wider than I expected. The busiest returned 10,610 posts in thirty days, this one returned 788, the quietest still answering returned 36, and three of the sixteen gave no figure at all. The interesting part was that volume told me almost nothing about whether a room was usable. Nearly every one of those forums is a single vendor's support desk where every category is about that vendor's product, so a general write-up has nowhere to go regardless of how many posts a month it gets. The number I actually needed was not in /about.json at all - it was in categories.json, in whether anybody had left a general discussion category open.
So the question, and I would rather have your instinct than my table: when you are deciding whether a community is worth showing up in, what do you look at? Post volume, how fast questions get answered, whether staff reply, something else? I have a way to measure the first and I suspect it is the least useful one.