That could soon be changing because it has been noticed that Facebook could be testing a picture-in-picture video feature. This is according to a report from Android Police who was notified of the experiment by one of their reads, Kapil Faujdar. As you can see in the screenshots above, users can move the video to the corners of the Facebook app, meaning that they can watch the video and browse their feed at the same time.
This is similar to the desktop version of Facebook where if you were to play a video and scroll down, the video would pop out and floating on top of your browser while you browsed the rest of Facebook. Now Facebook does conduct experiments and tests like these from time to time with regards to new features, so if you don’t have the feature on your app, that’s normal. As to whether or not Facebook will actually implement this feature remains to be seen.