1. Limitations of Mobile UI
Facebook isn't just one tool, it's dozens of tools combined. On a web interface with PC-browser sized real estate and mouse/keyboard interactions, you can pack a lot more features onto an app and not sacrifice too much in the way of usability and complexity.
With mobile, you have to make sacrifices due to limited screen real estate. As you pack in more features into any single app, you start having diminishing returns due to increased complexity.
2. Desired Tool
So considering the limitations, you have to ask: what function does the user desire in the moment?
Consider some of the FB mobile applications: Pages Manager, Camera, Messenger.
They're all quite unique from one another. On a web interface you might be afforded the luxury of building them all into the same app with a common entry point and framework.
But in mobile, developing all those functions into a single app would be complex due to the 1st point, and unnecessary due to the 2nd.