A school of thought in web development believes the web to be the next frontier in application development. For them it makes sense that websites like this feel and act like apps, both as an end-user (animations, transitions without full-page reloading, rich dynamic content, etc) and as a developer (client-side state management, frontend/backend separation, with an API in between, modular application structure, etc).
Apps don't load in 10ms, but they also can support some offline functionality given their independence from the server. Overriding browser behaviour and managing your own loading behaviour makes sense, because the default browser behaviour is not the experience you're striving for; it's not the app experience. These people are usually those who have worked on large web projects too— the developer experience that web developers have built for themselves in "userland" (JavaScript) is pretty good, and has evolved a lot to have features that makes developing the exact behaviour you want easier, and correctly iterating on a codebase, quicker.
A separate school of thought wish websites stayed true to their origins as enriched documents, and think trying to evolve them in the direction of applications is counter-productive to their true purpose as interactive surfaces to get and send information. If I am just going to scroll and see a few pictures, why do I need to download anything other than the structure of the page and the pictures themselves? If all the building blocks are already there, in the browser, why do people need to re-invent them and ship them as yet more JavaScript?
What should a website be though? The fact there isn't consensus about this is indication that there really doesn't seem to be a clear answer.
Per the document-like school of thought, facebook.com just keeps straying further and further away from the ideal, but as far as as the app-like school of thought goes, the new facebook.com is a pretty remarkable achievement.