What you say about comments on reddit, you need to take into account the posts you are seeing on reddit are the popular ones. There will always be forgotten post with zero comments' the same with YouTube, there will always be videos with less than 5 views. We never seen the movie because they are se deep is difficult to find them. But they are there.
About the number of users, you are totally right. Steemit is little. But I think it’s growing. And this concept (rewarding users for interaction) should eventually be the standard.