We released a new add-on that allows visitors to leave comments on your Tumblr blog posts. Visitors can now leave a comment as a guest (without signing in), or they can sign in using their Facebook, Twitter, or Google accounts.
It uses the awesome service Disqus to do this. So you’ll need to create a free account with them to use the add-on.
Instructions can be found here: http://help.22slides.com/article/blog-comments