If your business or hobby has a Facebook Page, you should see this graphic on the top of your admin page the next time you log-in as an administrator:
If you choose not to opt-in at the time, Replies will be turned on by default on July 10, 2013.
Previously, anyone could reply to a comment, but it ended up going into the comment stream with all of the others, so if you wanted to reply to a particular individual, your post would be hard to match up with the comment that you were referring to even if you added the person’s name.
Now, you can reply directly to a comment as you can see in the following screen grab:
According to Facebook, the comment that is the most active will be shown at the top of all of the posts, thus encouraging more interaction between your visitors.