I’m just starting to try to understand how best to go about this too.
The admin dashboard in Discourse does offer some decent data, with indicators that show whether certain metrics are going up or down compared to previous daily, weekly or monthly periods.
One of the motivations for a question I asked earlier this week about the last_seen_date was that I was starting to try to get a better sense of how to define and measure how many active users there really are.
There seem to be a number of discussions on the Feverbee forum that are starting to dive deeper into this question as well: