Not so long ago Facebook added a feature where video started playing automatically as you scrolled through your timeline. I found this particularly frustrating and others have said to me that this has eaten away significantly at their mobile data allowance. What annoyed me most was that there wasn’t an option to switch this feature on it just happened. Facebook have also not made it clear how to switch this feature off.
Your options are to play the videos whatever connection you have, and I’m guessing if you are reading this then that is not one of your favoured options. The other two are wifi only and off. This will certainly save your mobile data allowance and also give you control back as to when you want to play the videos.
Here in this video you can see the options on your iPhone and iPad.