Facebook SDK for .NET Core
So after not finding a solution, I've decided to develop my own some time ago. You may find here: https://github.com/developer82/FacebookCore
I don't know why you got a -1, but I am facing the same issue. Therefore for now I am going to use the OAUTH provided by .Net then direct requests to the graph API...
The other option I know would be to have a look there: https://github.com/Microsoft/winsdkfb
And there for some more advanced features: https://github.com/Microsoft/UWPCommunityToolkit/tree/dev/Microsoft.Toolkit.Uwp.Services/Services/Facebook
I am thinking about porting a SDK to .Net Core, but that may be too much work!