I am enamored with the idea of SDF, and I think it is an important part of computing history and the present. That being said, I am curious as to whether anyone actually finds it useful—aside from the fact that it hosts the instance!!
I used SDF for pretty much everything for a while: usenet, email, shell, and hosted my gopher site here (until I deleted it a year or two ago). These days I'm mostly on SDF's Mastodon instance and (occasionally) Pixelfed.