I find the FB groups are good for organising events, and general chatting, but don't provide the same user feedback that the forum does. its very hard to find information that has been covered in the past. i.e. try finding a conversation that was had 2 years ago on a particular topic? build diary's need to be maintained by an individual on their own page and get lost in the rest of the topics.
I have a FB build diary and am a member of a few GT40 groups, but its not the same as a forum.