Did that before we wrote the Plugin for a project we are currently working on. Some of the interfaces are mobile only. Facebook does not have an official C++ or UE4 SDK (they do for Unity). The other issue is you cannot have multiple OnlineSubsystem. So if I want to use Steam, then I cannot add FB, Twitter, LinkedIn, etc… I am stuck with a single Social interface.
I am not arguing with you here, but merely pointing out what UE needs to look at. I would have been happy to use an existing UE interface and saved a month of work.
Thanks for the input everyone…