When developed the multiplayer part of WordRoom we discussed to use different authentication providers. But it was a lot of code to integrate each provider. And we wanted more than one. Instead we developed our own solution.
But just ask yourself, do you want to create an account everywhere with different username and passwords. I don't. When we discovered Azure Mobile Services it was an obvious choice. Mobile Service gives you authentication with Microsoft-, Google-, Facebook and Twitter account and it's for free. You only need to connect it to a database server. But we already had a database server in Azure. So i next version of WordRoom, you can authenticate with your Microsoft-, Google-, Facebook- or Twitter account.
With just a few rows of code you can authenticate the user with one of the authentication providers.
In a Windows Store application you just add the SDK, some rows in App.xaml.cs and one row where you want to authenticate the user.
user = await App.MobileService .LoginAsync(MobileServiceAuthenticationProvider.Facebook);