Facebook's increasingly ubiquitous automatically-playing videos are a pretty dirty tactic in its battle for dominance with YouTube, registering your view without you having authorised it and producing inflated view counts that don't reflect how many people have actually watched the video properly.
They're also auto-muted, often making auto-viewing them useless, and they can be a distraction and a big drain on mobile data.
Here's how to switch them off
Mobile (where they tend to be more annoying)
Open Facebook app
Tap this icon:
Scroll down to Settings
Turn off Smart Auto-Play and either select Never Play Videos Automatically, or if you're worried about data, only allow them when using Wi-Fi.
While you're there you might want to toggle video uploads in HD on/off.
Click this arrow icon in the top right:
Go to Settings
Click Videos in the left menu
Select the dropdown menu next to Auto-play Videos and choose Off
Done! Your brain will thank you.