FB video is abysmal. It's surprising how such a bad video player can exist in 2018. Open the same video on facebook and youtube and the difference is clear. Facebook compression is so garbage that a 1080p video looks like a 360p video. AND still manages to buffer 20 different times. (They also just added 1080p video support a few months ago, before then every single video would get downgraded to 720p. Shoot in 1080, 2k or even 4k facebook would somehow manage to make it look like it's shot from a china phone camera from 1997.)
The thing is, on youtube when you upload a video, it gets compressed and stored on their server as one file. You can use any download manager to see that the video on youtube is one file.
On facebook however, for some reason (maybe to stop piracy or whatever) the video gets divided into segments after their God awful compression. If you use a download manager to download a video from facebook you will see many many different files to download and those are all tiny segments of the video.
It's the player that is so garbage that when playing and switching between the segments it gets stuck. Sometimes even goes to flash to stream video (that's how bad it is!)
Amazon Prime and Netflix both use a similar player where the video gets separated into segments, you will see on netflix or amazon some shots look awesome full HD and then suddenly next two seconds would be 240p and then two seconds later it will be HD again. But their player is actually good and can take it while FB player is a mess. And switches to flash player instead of HTML5 when it gets stuck and this just makes it even more of a burden on the browser.
You can see the photo I've attached and see the difference in quality (that is if fb's photo compression doesn't ruin it and make everything look the same). The top one is from facebook with the most amount of compression and took the longest to load (minus the bottom one), the middle one is youtube with less compression and took no time to load. The bottom one is from apple trailers where there's no compression. The last one took the longest to load but also shows the footage as it was intended without compression.