Here’s a good question and a really unhelpful answer.
Q: How long should a video on Facebook be?
A: “The best length for a video is whatever length is required to tell a compelling story that engages people.”- Facebook Product Manager Abhishek Bapna and research scientist Seyoung Park, (computer vision, machine learning).
Tweaking its algorithms again last week, Facebook announced a news feed bump for longer videos that are being watched. Other factors go into Facebook’s distribution decision as well, like whether viewers turn on the sound, go fullscreen, turn on high-def, among other things. And it considers whether or not a video is live.
In related news, Facebook is also adjusting ranking algorithms to promote authentic content “that people consider genuine, and not misleading, sensational, or spammy.”