FTA: "The average Facebook session lasted less than 90 seconds, according to people familiar with the matter".

How can you make money from people who are going to your site in 90 second segments? You have an ad in the beginning of your video, that's at least 5 seconds there. Assuming it's not skippable, that leaves 85 seconds at most for the rest of your video. The only kind of video that can go for 85 seconds or less are meme videos, and people who watch meme videos sure as hell don't have the patience to watch unskippable ads before watching memes.