Autoplay is fine on Facebook, it is an active response to your scrolling through the feed. It wastes bandwidth, but provides additional revenue for Facebook, which means there is at least some value for someone attached to it. But is stops when you stop.
The story is quite different for YouTube, which these days plays the next video immediately after the one you chose to watch. Yes, there is value for Google, but when people stop watching, often the videos keep streaming.
While many grown ups might leave YouTube videos running in a browser tab, the chief culprit is children and Apple TV / ChromeCast. Children (and half of the adults out there) think that turning off the TV stops the streaming of the media they are watching. It doesn’t – the iPod, iPad or smartphone that is feeding ChromeCast keeps streaming youTube, even while the screen is off. For hours and days. When the TV is switched back on, the child is pleased that YouTube is still running, saves them a few steps.
Prediction: Internet providers will create advertisements pleading people to stop their app instead of switching off their TV.