My most recent check-in changes the hook_fb API. I have a feeling not many users are implementing their own hook_fb, so I don't expect this to inconvenience anyone.
If you are, the new parameters are:
hook_fb($op, $data, &$return)
$op will tell your hook what is expected of it. See fb.module for the list of ops.
$data is an array containins a variety of things, depending on the $op. Most importantly, $data['fb_app'] and $data[$fb] will be set when these are known.
$return is a data structure which your app can set, for those ops which expect a returned value.