An app can provide notifications, integration and speed. An app can do a lot more caching than the app, and will never have to reload a version of its own code. There's places were apps are appropriate, and this is one of them.
I guess I'm just questioning the need for an app for all things, when many of the things are just as well served by a decent page in a browser.
One example - uploading photos straight from the camera. This is a html5 api but the browser support is appalling: http://caniuse.com/stream
More examples - integration with contacts? (i.e. appearing as an option next to call/text etc). Registering under the 'share' menu in Android? Registering as an Intent where appropriate? Ability to run background tasks?
I don't know if Facebook uses these but there's certainly good reasons for an service with it's wide spread of potential functionality to do so.
I really don't want it as a full integrated comms service!
It's a web page and I have a browser....