Facebook.com does a lot more than show a newsfeed, profiles, and notifications.

  - It contains a fully-fledged messenger application that supports payments, videoconferencing, sharing rich media etc
  - The newsfeed supports interactive 360-degree video, a live video player, mini-games in the newsfeed, and lots of other rich/interactive media
  - It's a gaming platform for 3rd party games
  - It's a discussion forum, groups management system, event planning UI
  - Photo sharing and editing platform, as well as live video streaming tool
  - A platform for businesses to have an online presence (Pages)
  - A peer-to-peer online marketplace (called "Marketplace")

And a dozen other things I can't think of right now. You might say "but I don't use all of those things". That's another tricky part, the fact that every user has a different "configuration" and different types of content in their newsfeed at any given moment, requiring them to be served different sets of JavaScript code.