Wow, that’s incredibly steady activity 
How did you avoid users leak to other platforms (facebook groups among others)?
I should have added in my post that my forums’ activity is also very tied to the sport popularity. There was a massive gain of interest in the mid-2000s, and it then decreased over time.
There are always multiple factors 
It’s a pity that we can’t write relative dates in Discourse. Like XX years ago, from a given date (or perhaps I just didn’t find how to do it).
Should newsgroups be considered as a forum?
In my case, the community was the same when it transitioned from newsgroups to phpBB.
Our community went through multiple migrations, and no doubt will go through other migrations in the future, perhaps to new communication software very different from what we know today 
I mentioned this multiple migration phenomenon here: