use fb comment box, hide wp's comment box

The Facebook plugin for WordPress attempts to hide the WordPress comment form using CSS on webpages where the Comments Box is active. You could add a CSS rule to set your comment form to “display:none” and hide from display.

this plugin actually does what you want out of the box to the annoyance of so many people. in fact, if you will make a survey of people who actually wants to replace the built-in wp comments with fb comments, instead of showing both wp comments and fb comments in a blog post, you will be amazed at the indignation.

I’ve tried it, it doesn’t hide WP comments so I had to simply disable WP comments in the backend and only FB comments are shown now.
Still thinking it might be better to integrate Disqus or something like that.