I don't know the details, because I don't use the Facebook mobile app or an iPhone, but the feature is advertised as "opt-in." In my wife's case (who uses an Android) she just noticed it start working one day and was surprised by it.

This is the official announcement of the feature. They emphasize it being "optional" and "opt-in" so, it appears it is off by default, and/or at least toggling that control is more clearly delineated in iOS, etc.

http://newsroom.fb.com/news/2014/05/a-new-optional-way-to-sh...