Facebook used to be server side PHP and it always was personalized. I think the distinction is more about the degree of interactivity. Highly interactive GUIs (the hell of jQuery developers) went to SPAs and are called webapps. Low interaction GUIs could be server side rendered with the technologies of 20 years ago even today. Most banking sites are low interaction and could be old style Java even if almost every one I see is a SPA now. The distinction is very fuzzy.
Why would an SPA do a "personalized experience" better than a traditional website?
Amazon is highly personalized and fits well into the standard web page paradigm. What about Instagram's "personalization" wouldn't work as a standard web page?
> What about Instagram's "personalization" wouldn't work as a standard web page?
Really nothing. But for most people Instagram is primarily a mobile application and comes with the usual experience of a mobile application. Transitions, fast reactions, no "whole page" reloads, etc.
Continuing this experience into the less frequently used browser version only makes sense, especially given the fact that Instagram's tooling allows for a relatively easy transfer (iirc large parts of the mobile apps are implemented using React Native)
Wikipedia and blogs are mostly for consuming content and it's the same content for everyone. Clearly a website. Instagram usually isn't super interactive, but it's extremely personalized, so it's much more like an app. Gmail also clearly a web app.