I'm not on FB anymore, but a mate of mine is and he often shows me the IG groups (hes also on here but is a propper lurkerI discussed the forum with them and they have been bitten by the facebook groups acting like dicks. One of the other customers googled the group and asked me if he had found the right one.

I think its easier to join a FB group with no intention of purchasing said vehicle, and just spouting crap.
Whereas a forum whittles down to the core audience of enthusiast.