How to use Facebook Authentication in ASP.NET Web Api MVC 6?

Question

I found these:

First Web API

Facebook on a WebApp

I'm trying to apply the knowledge from the above samples into a Web API of my own.

My clients will be Android and IOS native apps.

The clients already use the Facebook sdk to request the token.

But how can i use this token to authenticate my user?

I would love some example.

PS.: I use DNX Core dnxcore50 as framework.

EDIT:

I manage to use Facebook Authentication using the following: aspnet/Security/samples/SocialSample/Startup.cs

But the Response is a HTML to Facebook... I don't think that's how the WebAPI should bahave...

I think my question will be on this: Creating Backend Services for Native Mobile Applications