The gold standard for me is early LJ. It had a community with an active core group of a dozen or so, loads of lurkers but that core group made the community fun. And at the time, a fic posted to ff.net would always get a comment or more. I hate to tell at clouds, but the social media norm of the Like button has decreased engagement.
In between then and now I was in Tumblr, which was at first engaged, and then shifted with the general cultural change. This is across multiple fandoms and different media.