A video is given 1 view on Facebook each time the video has been played for at least 3 seconds.
Snapchat, Vimeo, YouTube, Facebook and other platforms have many different metrics. At Facebook, if someone plays your video for 3 seconds or more, you get 1 view to the video. This makes it very difficult to track the true number of Facebook video views, especially when videos autoplay in users streams. If 100 people scroll by slowly enough to trigger a video for 4 seconds and not a single person watches the video, the platform will report that 100 people watched your video.
People tracking video on Facebook will want to keep an eye on other metrics such as the lifetime average video viewed. This is an average length of video viewed by all video views.