No, it is not possible for the Comments plugin to connect to the on-facebook.com URL; it is only supported when it points to websites.
In order to achieve what you want (something on your site that can add comments to a post on facebook.com, right?), You need to get users to authorize the application you write that accepts their contribution and publishes it from their name back to Facebook via API
The actual implementation of this will vary depending on your exact requirements, but basically you want to:
Javascript SDK
, , (. : http://www.facebook.com/155965101083460/posts/155965104416793)
FB.api(
"/155965101083460_155965104416793/comments",
{message: 'This is great, thanks!'},
'POST'
);