Developers that have experience creating native applications are used to writing code that manages the application state and it's relation to UI interactions and data updates. In native-land, the UI frameworks are primarily the UI widgets and all of the application logic is written by the application developers in their chosen language. The corollary to this for web development is that HTML and maybe a few specialized graphics libraries like D3.js provide the UI thingies and the application logic (event handling, data fetching, etc.) is written in Javascript.

I'm not familiar with the Facebook website but I can't imagine that it needs any particularly complex application logic under the hood.