It can be disabled and you should use is. Saves a ton of data and speeds up for device.
It should be disabled by default. Absolutely the most obnoxious thing about FB.
..but that dark pattern helps achieve magnificent number OP mention :)
Unless Facebook is actively trying to slant the number of video views, auto-play should just be disabled. It really doesn't even need to be an option to turn it back on, because I doubt that many would want to.