public void GetFacebookFriendsTest()
{
//Arrange
var facebook = Mock.Of <IFacebookService>();
var facebookMock = Mock.Get(facebook);
facebookMock.Setup(m => m.GetFriends()).Returns("{json of friends}");
var sut = new FriendsController(facebook);
//Act
var actionResult = sut.GetFacebookFriends();
//Assert
//...do your assertions here
}