Facebook is going to start running pre-roll ads on its "Watch" videos next year. While you won't see your News Feed full of video ads, you will start to see pre-rolls, which will run for up to six seconds, on videos in Facebook's "Watch" hub. Recode reports: Facebook launched its Watch hub earlier this year, using "mid-roll" ads (another ad format Facebook tried to avoid for a long time). The fact that they have added pre-rolls -- the format used around the web and the one advertisers are most comfortable with -- should be read as an admission that the mid-roll ads aren't generating significant revenue for Facebook or the publishers putting video into Watch. Speaking of those mid-roll ads: Facebook now says they won't appear until later in videos and they'll only run on longer videos. It says the ads (it calls them "ad breaks") can't run until a minute into a video, and they can only run if the video is at least three minutes long. At first, the ads could run after 20 seconds and on videos as short as 90 seconds.