We don't have a way to automatically post back to Facebook when a user posts. In general, that's a very difficult task to do well because a reply, by itself, may not have much context. It also may not be the users wish to have that posted for them.
The Open Graph tags help make sure that if a user Like's the topic, then the preview for the page will look good in the user's feed. As always, have a good default image set in the Open Graph configuration area of JFBConnect since many topics likely won't have an image in them.
Finally, why users would post is up to you and them
Facebook is a great 'general' medium, but if you have a site that's focused on a specific topic, discussions are easier to start on-page. Additionally, as you're creating content, add links to the Kunena forum to discuss the article. That way, users know where you want them to comment.. although they certainly don't have to follow your nudge.
I hope that helps explain, but if you have any other questions, just let us know.
Thanks,
Alex
Kunena Facebook
