They do it weakly. It's difficult for dealers to set this up on their own (relying on the power of 3rd party API integration or even worse manually). Yes, the problem is having to upload the data. It's something I am very familiar with having done it within GA and web traffic attribution and then watching the implementation (pre-dates FB's attempts at it BTW) of sales attribution.
I'd be eager to know what you deem is a good benchmark or have you calculated it for dealers? I'll give Everson credit (where credit is due) and he does a lot of things right. However, I don't find those same cost / lead and cost / sales numbers as low across a large sample size. The bottom-line, there is so much fraud going on at FB, I'd be very, very skeptical if I were a dealer.