I'm in a couple of groups on facebook, and I understand that the reason people use them is because they're on facebook anyway. But I don't have enough limbs to count how many times I see the same questions over and over and over over ad infinitum. Like RV says, searchability is the big draw for a proper forum, things like signatures are great too because people can put basic details in there - like what model/year/engine they have - which is always the information people seem to miss when asking questions on FB. Forums will always be the quickest way to get the information you need when used properly, usually because the information you need is already here.