The WordPress blogging community has often had the need to utilize both comment input systems. Because of this, a lot of work has gone into "Facebook Comments For WordPress". http://wordpress.org/extend/plugins/facebook-comments-for-wordpress/
I think there is a lot the team could learn about what some of us need to function correctly from this prior work done.
I can relatively easily mod the code to make the native comment input form appear (and work). However, I've been unable to make the plugin not hide the prior, existing, (or new) WordPress native comments.
One step further would be to accomplish what this team has already ALSO succeeded at which would be to make WordPress report the correct total number of (combined) comments and to copy the FB comments into the WP database for archival reasons.
While I'm not a coder, I am avid in the WP community, and if I can be of any assistance in testing a code block, just ask :)