Facebook is not a social media network but also a video streaming site. People are uploading and sharing video on Facebook. Facebook has become a big competitor of YouTube. On youtube.com you can search any video by keyword but Facebook has not such facility. YouTube offers different types of quality for each video like HD, MP4 etc. on other hand people upload videos on Facebook.
You can change video quality setting from the setting section. Video will automatically play in HD format but if it uploaded in HD.To change this go to settings -> videos or visit the below url:
There you will be find drop down option for Video Default Quality. Select and choose HD. Done! now whenever you play video on Facebook it’ll automatically play HD if available.