Given the 85% figure - I would not be surprised if the "watched" metrics simply means it loads the video and begins playing it.
That means potentially only 15% of all facebook videos are actually engaged on.
We need more data on how long someone needs to view the video to consider it "watched." The way I understand it [1], is that facebook counts any video as watched so long as it shows - much like adwords impressions. Only if it goes equal to or past 10 seconds is it counted differently.
I read this as indicating that facebook video ads aren't particularly engaging.