Although using Facebook connect is really easy via Facebook Helper inside WebMatrix I thought why not making it even simpler by providing a ready to run sample!
So here it is! You can download the attached sample and replace the AppId and AppSecret inside _AppStart.cshtml
Then you can navigate to Account/Login and use the Facebook Login button!
PS : You can also navigate to /Facebook/Docs/startHere.htm to find out how to create and setup a facebook application!