I'm not lying, that's the actual size, something like 420k. It may have been a bit shorter playtime, perhaps 20 seconds (I didn't time it), but still, it was quite small.
Nobody said videos on Facebook are Blu-Ray quality. But you seem to have weird concepts about how big videos need to be to be good enough quality for a web page. Just as a test, I took an original high quality full-motion video of a concert, reencoded it with ffmpeg, audio codec aac, vbr audio quality 0.5, video codec x264, preset veryslow, cf 33, resolution 512x288 (half original size), 20 seconds. File size? 420k. Of course the video from facebook was darker and quieter, so one would expect it to compress better. If we give my sample concert clip an allowable size of, say, 550k, then I can up audio quality to 0.7 and cf down to 30. Either way, the resultant clip was fine, the sort of thing you'd expect to see on a Facebook wall.
Anyway, the key point is, Facebook feeds aren't loading you down with 50 meg videos, they're little couple-hundred-k clips, the same size as animated gifs. And while I haven't measured it, they don't appear to start streaming until you scroll down to them, and look to stop after you scroll away.