We’ve recently updated the Facebook application, and you will have to re-install. Follow these steps:
Uninstall the Silverlight Facebook client. You can do this in a few ways. First on Windows through the Add/Remove Programs or by right-clicking on the application while it is running (even though it doesn’t work) and choose ‘Remove Application’ – I recommend the Add/Remove Program control panel option. On Mac OSX, simply move the application to the trash.
Ensure you have Silverlight 4 RC installed.... (more)