From Facebook Developer Wiki

Does anyone have a .NET example of this?

This is a bug in setUserPreference since it has stopped being supported

Although the documentation says you can still pass this call, but you must include your application secret, passing an empty value ("") to clear a preference will result in an error.

Hi Oliver, thanks for reporting this. It would be great if you could file a bug so we can track this. Thanks, Pete -- 563683308 12:51, 9 October 2009 (PDT)