I don't really use it either. It might be better just to have a link that pulled up a popup list of similar posts, like tags and usernames do. I'm working on a Hubski Search at the moment, and I think that should be a better avenue for content discovery.
A link to a popup or a reveal of similar posts would be used even less. Not saying the placement is optimal, but having them visible lets you quickly scan if there is something you want to see. Think about it, if people can see it now, but don't use it regularly, having them take the effort to click on it only to reveal nothing click-worthy would very quickly train them to never click on it. I don't think hiding the posts is the answer if you're going to have that feature, imo.
You might be right. But, multiple column elements are difficult when we are dealing with so many different aspect ratios. I can't think of a good horizontal placement. Maybe at the bottom of the comments? I definitely don't want to get rid of the big images.
I don't think people are ignoring them, they literally aren't seeing them. I never notice them, perhaps they should be more evident but with an option to "hide"?