Giang,
The settings in the Social tab only are for the automatic comment box added into Joomla articles. If you have that comment box in EasyBlog from JFBConnect, you're probably using the {JFBCComments} tag to insert that box. If so, you just need to update the tag with the width parameter, like {JFBCComments width=500}.
I hope that helps, but if you have any questions, just let me know.
Thanks,
Alex