Honestly, I've got no idea. I have my own Tumblr blog with a ton of pals who read every day. But the short items aren't very good at starting conversation.
Not even sure I'd have comments if I were you. Instead, I might focus on making things easy to share and rate (e.g., 1-5 stars, like / boo!). Allow your comments to reflect the ethos of your chosen short form.