Facebook has been willing to run ads in the middle of videos, but it has historically been reluctant to put ads in front of videos and irk people just trying to watch a clip that surfaced in their News Feed. However, it might be willing to break with tradition in a limited way. The social network plans to test pre-roll ads in the Watch tab and other spaces where you "intentionally go to watch videos." You won't see pre-video ads while you're scrolling through your friends' updates, in other words, but you might when you sit down for a viewing. The trial will start in 2018 with 6-second clips, with Facebook studying the results to see what works best for different audiences and shows.
There will be some changes to how videos are prioritized in your News Feed. Facebook is updating its ranking system to increase the distribution of videos that people are looking for (such as through search or a company page) and those that people keep coming back to watch on a regular basis.
In both cases, Facebook's aims are clear: it wants to both make more money from videos and encourage producers to keep making videos. If you know your kitten compilations will both rake in more ad revenue and reach a wider audience, you're probably going to keep posting those clips. The ads could certainly be nuisances, but Facebook is betting that you won't mind if those and the News Feed tweaks lead to more of the videos you'd like to see.