So you're trying to play a video from the Facebook app? Can you tell if the video works on your computer browser? Or maybe try to access Facebook on your mobile browser, and see if it works there.
What format is the video? Try using MX Player to play it. I don't use Facebook so i don't know if it tries to play it within the app or through the stock video player. But try either setting MX Player as default app or downloading the video and playing it in MX.