One limitation in Open Graph that it accepts only real-world objects like: Movie, Author, News and Band. I would like to use with more non-real-world objects like a post, question or a comment.
There is one entity that might fit in such use which is the Article, but using it for both a post (e.g. blog post) and it's comments would be a little inappropriate.
However there is another not very real-world type which is News I though that this might fit in such use case: Blog Post as Article and it's comments as News, would this be ugly?
The Open Graph allows you to create any objects and actions you want.
In this case, create a custom object which represents the object the user is commenting on and a custom action "comment". Add a custom property of the comment action which stores the text of the comment.
For posts, you can create a custom object "post" and a custom action "authored" - resulting in stories like "{user} authored {post title}" - you could use wrote, posted etc as alternatives to authored.