Also, is there any centralized forum that is more active than the Threadiverse (Lemmy/Piefed/Pylova/Mbin)?
Fecesbook, Shitter, and LickedIn came to my mind first.
Then there are Reddit and Tumblr and a couple more.
But all of these are worse experiences than the Fediverse. And I believe that's because the Fediverse is federated.