Hacker News Reader

If you take a look at the announcement post for the framework, you can see some discussion about that: https://code.fb.com/ml-applications/horizon/

One application that they mention related (at least loosely) to social connections is deciding whether or not to deliver a notification to a user.

That makes sense. At Twitter, I redesigned the notifications system to explicitly allow for ML based decisions such as this.

Any idea on what the objective function for such a thing would be?

I'd imagine that a company which exists entirely to collect huge amounts of data might have some interest in useful ways to automatically process that data.

Most likely to make the News Feed more engaging