Of the three examples you gave -- Facebook, YouTube, and Gmail -- you could make a good case that only Gmail is actually an application: it's a mail client that runs in the browser. YouTube is arguably almost as document-centric than Hacker News is; it's simply that the documents in question are built around embedded videos and their comments. Facebook is somewhere in the middle. It's accreted application-like qualities over the years, but at its core it's assembling a list of text and image items -- i.e., documents -- into a master timeline.
I'd submit that an awful lot of web sites that people spend time on have been made very JS-heavy and "application-y" despite being fundamentally document-driven. In part I'm sure this is due to the rise of native mobile applications -- have your server just use a JSON API and make your "web page" a JavaScript application that talks to it the same way your iOS and Android apps do. That development model has become the hammer web developers are using to hit an awful lot of things that maybe really aren't that nail-like.
I'd submit that an awful lot of web sites that people spend time on have been made very JS-heavy and "application-y" despite being fundamentally document-driven. In part I'm sure this is due to the rise of native mobile applications -- have your server just use a JSON API and make your "web page" a JavaScript application that talks to it the same way your iOS and Android apps do. That development model has become the hammer web developers are using to hit an awful lot of things that maybe really aren't that nail-like.