http://lifehacker.com/how-to-turn-off-facebooks-auto-playing-video-ads-1563822464 or https://www.yahoo.com/tech/how-to-disable-autoplay-videos-on-facebook-82823336266.html
For iOS it seems that they even added an actual "Off" option there as well recently.
Thanks! I've been looking in the settings on the app itself, not the iOS settings.