This is a question I've seen asked on other forums I'm a member of, and nowhere have I seen a conclusive answer.
I think one of the problems is that there is a greater tendency for people to be "content consumers" rather than "content creators". Of course, by definition everything on a forum is user created content but I sometimes wonder what the ratio of is between those who just read the forums and those who actually contribute more than just clicking the "like" button.
For myself when I comment on a "what are you watching/reading/listening to" topic I try to put more than just the name of the series/manga/song in the hope that it might attract a comment, sometimes it does, sometimes it doesn't.
I guess none of that really answers your question, but then it's a tricky question to answer.