http://developers.facebook.com/docs/reference/fbml/silverlight/
FBML has been deprecated. Starting June 1, 2012 FBML apps will no longer work as all FBML endpoints will be removed. If you are building a new application on Facebook.com, please implement your application using HTML, JavaScript and CSS. You can use our JavaScript SDK and Social Plugins to embedded many of the same social features available in FBML.
What does this mean to silverlight apps?
FBML is a Facebook-interpreted markup language that used to be how apps on Facebook worked – it hasn’t been possible to create FBML apps for about a year now.
It’s also unlikely that you’d have had silverlight content in an FBML app (but it’s possible) – if your canvas mode on the app is ‘iframe’, you’re not affected.
if its’ ‘FBML’ – you’ll need to reimplement your app using an iframe connection to your server instead of the Facebook-interpreted version