I found a very good WordPress plugin which could replace the built-in WordPress comment box with Facebook comment. It is developed by Fat Panda. Search the term Fat Panda in the Add Plugin page.
It works well in blog post but it does not replace the built-in comment box in attachment page. I am using the Twenty Eleven 1.3 theme. Let’s see if this could be fixed in later version.
Reference: Fat Panda – Facebook Comments for WordPress