While I am still investigating how integration between multiple social networks and Silverlight is best achieved I have found that MS have release a Silverlight 4 client for Facebook so at least I now know this can be done at a conceptual level. The client can be accesed from the link below:
More details can be found on the AllFacebook blog
and finally the key ingredient the facebook SDK includes assemblies from Silverlight, MVC, Web and Winforms!
Give it a try!