Be one of the first to get the Microsoft Silverlight 4 Beta Client for Facebook!
Test the Silverlight 4 developer preview application for Facebook which combines the many compelling platform capabilities of Silverlight 4 in a rich out-of-browser application within the familiar context of Facebook. 
Enjoy seeing photos, reading feeds, learning about events, and posting things on your friends’ walls while sending and receiving mail from your Facebook account in this appealing and user-friendly environment.
If you haven’t done so already, you must first download the Silverlight 4 beta in order to install and use the Silverlight 4 Beta Client for Facebook. Download the version that is right for you from these links:
One final note: When Silverlight 4 is released you may be required to uninstall and then re-install the plug-in.