The only information the provide on this is from Facebook Help:
How can I make sure my videos are high quality?
The easiest way to optimize the quality of your videos is to upload HD
If you're exporting your video from editing software (ex: Final Cut
Pro, Avid, iMovie) we recommend these custom settings:
- H.264 video with AAC audio in MOV or MP4 format
- An aspect ratio no larger than 1280px wide and divisible by 16px
- A frame rate at, or below, 30fps
- Stereo audio with a sample rate of 44,100hz
Your software's guide should include information about exporting video
with custom settings.
Keep in mind that videos must be less than 45 minutes long and smaller
than 1.75 GB. The longer your video is, the larger its file size will
be. This may affect the quality of the video.
Beyond this, it's impossible to say what they do to the videos uploaded to them. Chances are they will re-encode all videos regardless of what comes in, to ensure they are exactly what they need them to be.