Facebook-WordPress comment/feedback integration
There's a bunch of facebook-connect enabled WordPress plugins that might help you achieve this.Here's a couple:
yoavf answer is good. A list including those plugins is maintained in Facebook's wiki:http://wiki.developers.facebook.com/index.php/Facebook_Connect_Plugin_Directory
http://dentedreality.com.au/2008/12/implementing-facebook-connect-on-wordpress-in-reality has a novel approach that isn't on that list.
you need to go to the source of the blog, figure out how the comment form is posted (POST or GET) then you can use mock ajax documentation here. then inside the facebook app, you build a form similar to the one for the blog comment. make sure all the ids of the respective form elements are identical. then on your submit button you have a link like this.
<a clickrewriteid="dummy" clickrewriteform="comment_form" clickrewriteurl="blog_form_action">Submit</a>so dummy is the id of an element that will recieve any output.
comment_form is the id of the form that will be posted to the blog comment processor.
blog_form_action is the url in your blogs form action attribute.
if things work, the form comment_form will be posted to your blogs processor page so that the comment submitted from facebook will appear on the blog as well.
if the blog uses GET as its method, you just make sure that blog_form_action contains the query string
hope that helps!