HD and SD, never re-encoded
Facebook publishes a video as two progressive MP4 files — an HD pass and an SD one. Both are listed, HD first, and whichever you pick is streamed straight through: nothing re-encoded, resized, or watermarked on the way past us. The file you save is the one Facebook serves its own player, which is the whole of what an HD Facebook video downloader can honestly offer: the HD pass that was uploaded, never an upscale of the SD one.