login via social Media

I am trying to add to my application a login via social Media (Facebook, Google+ and Tweeter) . I searched the web for a xamarin.forms solution but all solution I found use xamarin.droid and xamarin.ios I didn't found any solution based on xamarin.forms . Any Idea or help about a Library that work with Xamarin.forms?
I tried xamarin.Auth and xamarin.facebook.android and xamarin.facebook.ios .... all these library are native