> None of these actually require JS.
Can you imagine a facebook doing a page reload/refresh every time you click to like a post? Or without loading more content on demand every time you scroll the page?
You're stuck thinking about Facebook as if it still had long lists of posts with infinite scroll. The UX would be completely different when the design constraints are different.
For instance, instead of infinite scrolling, you might show one post at a time with clickable previews of the last and next posts. A like doing a full postback isn't a big deal with this approach, particularly with judicious use of anchors. Certainly not as slick, but perfectly usable.
Agreed on built-in browser behaviour though. Chrome pushing more input types a few years ago was a great thing.
(I suppose the companies like IS because it has an addictive property, but I suspect me and others who see through that don't like it at all. Relatedly, the other popular concept of a "feed" also conjures up images of farm animals munching away at a trough; perhaps that is the real intent...)