Facebook videos under 30 seconds loop continuously.
Videos will loop continuously up to approximately 90 seconds.
Looping also applies to replays of Live videos under 30 seconds.
Videos over 30 seconds won’t loop. Once the video has played through, the video’s thumbnail will appear with a play button in most cases or auto proceed on to the "next video."
Call-to-action buttons can be added to video ads, and appear under the video. (Info sourced from Facebook help page.)