Visual art gets the most engagement in any fandom space, at least on microblogging social media like twitter and tumblr. But there's certainly no shortage of engagement for fanfic on AO3 and the like. I haven't written a fic in years and I still get steady emails about kudos on my stuff. When I've posted art it's a bunch of interactions up front but then a sharp drop-off, I'm certainly not getting any Tumblr notes for anything over a few months old. But then I've never gone "viral" (and don't want to), maybe that would be different.