As a blog platform, tumblr can be too minimalist sometimes. I really wish commenting was a feature built into it. There are ways to help yourself: Adding disqus to your tumblr is what I did, and there are a few manual steps, but it’s nothing terribly difficult.
Funny enough, more people comment on the re-blogged articles that show up as facebook notes than on tumblr itself. I guess that worksforme, too. Just need more friends 🙂