I didn't have a look yet but I guess it's a combinaison of removing all secondary content (ads, comments, etc.), compressing images, cache all the assets in all Facebook CDNs and saving them offline based on your favorite and popular news websites among your friends on a background process.
My understanding is that Facebook is increasingly moving towards a functional representation of state akin to event sourcing and gleaning performance via the "micro services" approach and one way data flows the paradigm facilitates.