gistfile1.cs

string productId = "YOUR_WINDOWS_PHONE_STORE_ID";

string facebookAppId = "YOUR_FACEBOOK_APP_ID";
string redirectUri = "msft-"+productId+"://authorize";
string scope = "public_profile,email"; // What you want to fetch from the facebook user
string responseType = "token"; // Other response types possible

UriBuilder authUri = new UriBuilder("https://www.facebook.com/dialog/oauth");
authUri.Query = "client_id=" + facebookAppId + "&redirect_uri=" + redirectUri + "&scope=" + scope + "&response_type=" + responseType;

DefaultLaunch(authUri.Uri);

async void DefaultLaunch(Uri uriToLaunch)
{
    var success = await Windows.System.Launcher.LaunchUriAsync(uriToLaunch);
}