The Facebook app for iPhone and iPad features an auto-play feature for videos frequently causes a lot of users some data usage grief. It works by auto-loading and playing videos as you scroll through your timeline, whether or not you want it to. If you don’t like this behavior, there’s an easy way to turn off auto-play. It’s something we’d highly recommend doing if you don’t have an unlimited data plan or live with restricted Wi-Fi. And as an added bonus, it may save you a bit of battery life too!
All the instructions are here ~ http://www.imore.com/facebooks-auto-play-videos-eating-all-your-data-heres-fix