I didn't really notice much of an exodus from the forums I was on till around 2008. I think ATF was still pretty vibrant when I started posting again ~2012. ATOT wasn't it's usual hot mess of infinite posts, but that was okay with me since I spent 95% of my time in the tech forums.Reddit was the death blow to many forums. Since you know, Reddit is effectively a forum.
So many internet website forums I post in are dying, I think internet forums kinda peaked ~2005 and have been on a downward trend since (reddit, facebook)