I usually throw Medium articles into Outline[0] for readability. There is also Archive.today[1] where the article is usually mirrored already. My only issue being that Archive.today could go down since it's expensive to run such a service.
> GitLab and GitHub
As for Facebook, I think the only way such a frontend to that would work is to use a Facebook account to scrape walled garden content and present it externally to users. I'm not sure Facebook would like that however since it hurts their bottom line. They would probably code against such tools, and then we have a whack-a-mole scenario.
As for Gitlab & Github; I see no reason for a frontend since they don't hide things behind a login prompt and their UI is pretty intuitive (albeit a bit bulky and bloated).