The Facebook package has been migrated to the Universal app stack, however the Facebook.Client package has not. The Facebook.Client version for Windows Phone 8 targets the Silverlight version of the APIs, so it won't auto-magically convert to supporting Windows Phone 8.1 Universal projects. Someone is going to need to port it.
The source can be found here: https://github.com/facebook-csharp-sdk/facebook-winclient-sdk