I too have just started receiving the same error. I have reauthorised the account, and also removed the app from my facebook settings to start a fresh auithentication, but this still fails. I have even reinstalled Lightroom, to no avail. HELP!
P.S. I too am a Mac LR 4.1 user
Problems solved! This was sourced from a comment on the interweb
"Debug:1169: Error reply from Facebook: (#240) Requires a valid user is specified (either via the session or via the API parameter for specifying the user.
I found out the fix.
Access your facebook fanpage when you are logged into your personal account. There is a new “voice” section and make sure “You are posting, commenting, and liking as the fanpage” not as your main profile. This will stop the error #240.
Hope that helps"
Message was edited by: gervank