None of them is a web app. All of them want to provide certain views on their content that are uniquely identified by their URLs. This is especially true for Twitter and Reddit, which both aim to have their views on tweets and threads to be indexed by search engines.

Facebook is probably the least clear one, since they put great effort into eliminating discoverability and accessibility from "outside", i.e. without a Facebook account. They're basically the AOL of today, which was a classic application back then that tried to contain a proprietary "web" of stuff in itself, only accessible if you ran their "browser" application, had an account with them and were logged in, and that "AOL Internet" of course had its own search mechanisms, just like Facebook has its own sophisticated search. However, they're not fully closed-up, they just try really hard to nag you into having an account, which is why I'm inclined to still put them in the "website" camp.

However, all of these web sites surely contain certain parts that are in itself "web apps" and that are pretty much isolated from the basic parts of the site (albeit possibly interacting with it at well-defined points through APIs). I guess that this is a logical result of the large engineering teams that are working constantly on what is basically the same big website - at some point, in order to scale out further, it just becomes economic and practical to effectively isolate certain features into mostly separated "apps".

It's really easy to throw the baby out with the bathwater in this conversation as well.

A more interactive Javascript-heavy UI can improve a lot of aspects of even websites we consider to be a server-side-rendered gold standard like a forum or message board.

For example, look at the right-sidebar scrubber on Flarum that lets you quickly scroll to arbitrary parts of a long forum thread: https://discuss.flarum.org/d/17745-flarum-0-1-0-beta-8-relea...

Or look at how clicking a submission on Reddit's new UI opens it in a floating modal window so that you don't lose track of where you are when you click out of it. It makes opening submissions feel cheaper/faster to me where, in the old UI, I tended to open everything up in a new tab.

There's a more thoughtful consideration that needs to be had here than just "webapp or website?" Else you're back to cargo-culting.

"Is Reddit a web app? Is Twitter? Facebook? They all started with server-generated HTML, all kinda feel "document-like", all of them involve scrolling (if that means anything.)"

All three of these are examples of websites where JavaScript should be entirely optional. With the exception of the realtime chat functionality in reddit and Facebook, there is very little that any of those three platforms couldn't do exclusively with server-side-rendered HTML and CSS. It might be ugly, but it'd be usable. Any client-side scripting should build on top of that and implement the fancier features.