If you have stayed on a video for at least three seconds, it signals to us that you are not simply scrolling through feed and you’ve shown intent to watch that video.
Hi, my name is Matt Pakes and I’m a product manager at Facebook, responsible for video products.
Matt Pakes

Bullshit, If I “intend” to watch a video I will unmute it, not sit there for 3 seconds. This is a VERY terrible AND LAZY way to program something of such an easily FAKED thing.

Watching a video is like checking a checkbox, just because I hover over it for 3 seconds to figure what it is does NOT mean I INTEND to check that checkbox.