Facebook's motivation is not that users of the site want it. It's that when they autoplay it they get to lie to advertisers and claim that "users engaged with the video content for x seconds," and charge them accordingly.
Every person I see using Facebook scrolls past the vast majority of videos in their feed as fast as they can because they don't want it to start playing. And then when they do find one they want to play, it's this race to pause the video and rewind it to the beginning as they make it full screen and start it over so they can see the whole thing. Then at the end, they have to stop the auto-scrolling to the next one so they can go back and see the funny part they liked, or whatever. It's so ridiculous.
[0]http://www.adweek.com/digital/every-major-advertising-group-...