Facebook is testing a new feature where videos in your news feed will automatically begin playing (with the volume turned down). That's potentially bad news for anyone using the Facebook mobile client.
The feature is currently being tested on the desktop client, and Facebook says it will begin rolling out on mobiles in the "coming weeks". Right now, only videos from individuals and musicians will play; ads won't. The videos are silent initially, but go full screen and add audio if you tap or click on them.
To my mind, the autoplay move is a really bad idea on mobile. Even if you don't want to watch a video, you'll be downloading at least some of it, and that will waste data. I've reached out to Facebook to ask if there will be an option to disable this "feature", and will update if I hear anything. Again, this is being rolled out gradually, so you might not see it for a while (I'm hoping I won't see it at all).