The Facebook app and Facebook Online Directory are two completely different things and you need to log in to each seperately. The app data shows basic wall posts, messages, etc and doesn't cache very much data, while the directory service downloads contact info, personal data and profile pics for ALL Facebook friends and adds it to your phone Contacts, and the size of the stored data is related to how many FB friends you have. Best guess is that each FB friends need about 80-100k storage.
It should also be mentioned that simply using Facebook for HTC Sense does result in bloating to an extent, but does not cause the same issues as the Facebook Online Directory.