I was talking mostly about generalist forums. Indeed topic-specific forums are still active, but that's not something OP's forum, Reddit or the Threadiverse tries to do.

They’re not as big as Reddit but some have an order of magnitude (at least) more users than Lemmy does.

I just checked https://forums.somethingawful.com/index.php , it shows 2,369 users logged in. A single instance like Lemmy.world has 4880 daily active users.