On Thursday, Facebook announced longer videos watched through will be given a higher priority in the ranking of the News Feed. Hidden within a jargon-filled, 411-word News Feed FYI (a blog dedicated to publishers and other Facebook Page owners), Facebook shared a change to the algorithm that addresses completion rate:
As we continue to understand how our community consumes video, we’ve realized that we should therefore weight percent completion more heavily the longer a video is, to avoid penalizing longer videos.
So it's not just about those casual moments; those six-second (RIP Vine) puppy videos. It's about you essentially experiencing television when you're on News Feed. That's not unlike YouTube, which categorizes video based on watch time.