FB notification is not technically out for consumers. So it is still in beta mode! Thats prolly why you sometimes get notifications.I'm running OS X 10.8.2 developer preview, and I was unaware that you can get Facebook notifications... but it's weird 'cuz I sometimes get them or non at all.
Probably the least surprising thing I've read all dayI upgraded to 10.8.2 pre-release and since then I have started to receive my first ever Facebook notifications in notification centre. Before 10.8.2 I hadn't received a single notification with or without the Facebook website open.