By default Facebook video is set to automatic Play option once you hover over a video on your browser.
This option can drain your internet data unnecessarily because it plays any video you come across including the ones you don’t want to play.
To turn this option to “Off”
Goto “Settings” in your profile, and click on “Videos”
Under “Auto-play Video” choose “Off”
Viola, only videos you press the play button will now play on your Facebook.