It is worth pointing out here that the UI is not what you would expect. Most often, with a Xamarin Forms application, the UI is constructed from the PCL. Moreover, the likes of Facebook and Twitter provide their own login user interface. This is part of the remit of OAuthAuthenticator.
This means that the PCL UI for the content page is empty, and we rely on a custom renderer to produce the user interface with a callback to the PCL once the login has been performed.
A very simple implementation of a Facebook login (as supplied with the example code for this chapter) looks similar to the following screenshot when executed. The example is using the Windows Phone emulator under the Windows directory, but the same UI is produced for Android and iOS.
The reason why the UI is the same for the login is that it is supplied by Facebook as an embedded web page through the authenticator service.
