Additionally, Facebook knows which apps you are using anonymously, which could have problematic implications when incorporated with Graph Search.

this is exactly what it is. They can still target ads and understand your preferences and behaviour.

The horror!

> I suppose it still tells that service that I have a FB account.

Which barely more invasive than telling them you breathe air, no?

This is the big question. I assume it is telling Facebook and NOT the app developer, so basically it's all win for Facebook and all loss for the developer who implements it (other than attracting the few extra users who care so much about their privacy that they won't divulge their Facebook data).

I'm curious what happens to functions inside the app that need identity. Do they work as normal but the app developer can't see the information? Or does everything the user does prompt for a login, so the app developer can be pretty assured that nobody will go for any length of time using the app before they give up and log in?

What meaningful information can a service glean from that when probably 90% of the western internet using world has one?