FACEBOOK have managed to auto set videos onto AUTO play which uses non stop data on mobile devices.
I had not changed any settings and for first time ever was reaching data use levels, checks show auto play has been switched on. I did not know that was even available.
If you open FB and see a video start to play as you scroll passed it they have switched the auto use on without your permission.
To change it on IPhone
go to settings page and scroll down to FACEBOOK and open in settings, you the see an option to open settings under the Facebook logo, OPEN that and then you see auto play half way down, enter that and click box off. Hope that does it, mine just sorted and it was permanently on FB switched it on as I did not even know it existed!!!