But now that I think about it, I also see them for payment forms, where the loading time is usually short. Thinking more about it, I think they annoy me because they fake having loaded something useful. But there is no interaction yet, nothing I can do. A spinner is truthful, a skeleton is a lie. Interesting, I had never thought about this before ;)
Users want speed, using a spinner or a skeleton won't fix a slow response. The best approach is to CDN everything possible, locally cache everything possible, and show a simple spinner IMO. Spinners are just there to say "wait, please, something is happening". The response time needs to be reasonable, that's all. Users aren't dumb and they know a skeleton is only a (dishonest?) illusion
Because just look at HN itself; a comments page is a long list of content, not dissimilar to the examples given of Facebook, but it loads instantly with only a small spinner in the tab bar that resolves in less than 200ms. No skeletons or spinners are actually needed on the page itself.
I'm convinced that most sites would work faster if the pages are just rendered server-side. I don't believe it would cost more server load, and I don't believe the extra server side cost, if any, outweighs the vastly improved client performance. Plus it can be scaled, especially with modern-day techniques like CDN workers / lambdas; they could put the templates and final rendering into those edges, close to the end user, and only pass data between their datacenters and the CDN's edges. From there on out they could make the pages partially dynamic.