I am using Ultimate Facebook plugin and have selected Use Facebook Comments, as this is required if comments made on my blog are to be also published on my Facebook page.
The standard blog published comments and comments entry box are responsive and reformat well on mobiles tablets etc.
The Facebook published comments and comments entry box are not responsive and do reformat correctly.
In the UF Facebook settings you are able to specify a width for the FB comments entry box, I have tried leaving this blank but does not make a difference.
There must be some custom css that can be added I guess.