My problem is a little differenr, my Facebook videos automatically play sound whenever I use the microphone on the keyboard whether or not I'm in the Fb app
Try using the iCleaner Pro app to remove any preferences that are still left on the phone. Tap on the plus sign at the bottom left and then preference files and delete any files that have to do with the Facebook tweaks. Slide to the left to delete. I always do this when I remove a tweak.