I would say a substantial amount of time is spent by most users using applications. For instance, Facebook might eat up more than 50% of some users time on the internet, which I think falls under the category of application. Or Youtube, Gmail, etc.
There may be a small minority or users who spend a lot of time reading documents (and I applaud them!), but most users are interacting with web applications.
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 definitely spend more time using applications, and for the general population I'd bet even more. Webapps are what people are flocking too and that's where the industry is heading. Depending on your definition of application, arguably reddit, gmail, youtube, Stack Exchange, and Facebook are all webapps.
I'd argue that forum sites are primarily document based.
We've managed to emulate the functionality of a 1990s (or 80s!) dial up BBS and scale it up 100x (now instead of a thread having 200 replies it may have 20k replies), at and we are only needing a few thousand times the bandwidth and CPU power to do it!
Do you spend more of your time on the web using applications than you do reading documents?
I kind of doubt it. Maybe. But probably not.
It may be that you spend more time making web applications than documents. That's understandable.
But is the web primarily for you as a developer, or as a user?