András Velvárt discusses how basic Facebook features (such as logging in, posting to a user’s wall, etc) can be accessed from within your Silverlight application.
Source: Dotneteers
Social networks have grown to be a really important factor to consider when you are building any sort of publicly available application - and the biggest one out there is almost certainly Facebook. According to some of its own statistics, it has more than 500 million (yes, that's no exaggeration) active users, half of which log on to Facebook every day. Those 500 million users each have 130 friends on average. That means, that getting any one of those 500 million to use your application, might lead to 130 more learning about it, 130 more people each having 130 more friends they can bring and so on. You will probably agree with me, that this alone is a pretty strong argument for connecting your application to Facebook.