Anyone else who gets stuck on this, you can authenticate a user by simply setting the AutCookie session with:

System.Web.Security.FormsAuthentication.SetAuthCookie('Username', true);

Just to expand on my code, The user is Authenticated via the Facebook JS SDK.

So on my Login View page I have:

<script>
    window.fbAsyncInit = function () {
        FB.init({
            appId: 'MY_APP_ID',
            xfbml: true,
            version: 'v2.5'
        });
        // Additional initialization code here
        FB.Event.subscribe('auth.authResponseChange', function (response) {
            if (response.status === 'connected') {
                // the user is logged in and has authenticated your
                // app, and response.authResponse supplies
                // the user's ID, a valid access token, a signed
                // request, and the time the access token
                // and signed request each expire
                var uid = response.authResponse.userID;
                var userAccessToken = response.authResponse.accessToken;


                $.post("/umbraco/Surface/BookingAgentSurface/FacebookLogin", { accessToken: userAccessToken });

            } else if (response.status === 'not_authorized') {
                // the user is logged in to Facebook,
                // but has not authenticated your app
            } else {
                // the user isn't logged in to Facebook.
            }
        });
    };


    (function (d, s, id) {
        var js, fjs = d.getElementsByTagName(s)[0];
        if (d.getElementById(id)) { return; }
        js = d.createElement(s); js.id = id;
        js.src = "//connect.facebook.net/en_US/sdk.js";
        fjs.parentNode.insertBefore(js, fjs);
    }(document, 'script', 'facebook-jssdk'));
</script>

And to render the Login Button:

<div class="fb-login-button text-center" data-max-rows="1" data-size="large" data-show-faces="false" data-auto-logout-link="true"></div>

So once the user is logged in, the Facebook subscribe event posts back to my SurfaceController with the userAccessToken, so my SurfaceController looks something like this:

    public ActionResult FacebookLogin()
            {
                string accessToken = HttpContext.Request["accessToken"];
                try
                {
                    var client = new FacebookClient(accessToken);
                    dynamic user = client.Get("me");

                    var memberService = Services.MemberService;
                    IEnumerable<IMember> ExistingMember = memberService.GetMembersByPropertyValue("facebookUserId", user.id);

                    if (ExistingMember.Any())
                    {
                        IMember existingMember = ExistingMember.FirstOrDefault();
                        System.Web.Security.FormsAuthentication.SetAuthCookie(existingMember.Username, true);

                         //USER IS LOGGED IN, REDIRECT TO ADMIN SECTION ETC
                        return Redirect(Constants.AppPaths.BOOKING_AGENT_DASHBOARD);
                    }
                    else
                    {
                       //USER IS NOT LOGGED IN
                        return "User not found ";
                    }
                }
                catch (Exception e)
                {
                    return e.Message + " " + e.StackTrace;
                }
            }