It is not necessary to have an image of a brand planted in your mind before a purchase. It only matters that you're guided towards the brand when you are ready to buy. Advertisers were happy with a "brand image" before because it was the best they could do. But these days, it seems almost worthless.
Cars are, for most people anyway, the kind of really large purchase that requires a lot more effort and deliberation, not to mention financing, than makes sense to do in one click.
Then, I can choose where and how I buy the car later. The manufacturer would benefit from this no matter how I buy the car. Keeping these "shark-free-showrooms" stocked with demo cars, coffee, pastries, and branded swag would just be a general marketing expense.
I think they have the data, but maybe not the means to mine it. I'm guessing that people are interested in buying a new car at important life milestones and when replacing an older vehicle. It's pretty easy to find those milestones ("here's pictures of our new baby", "Grad 2015, whooooo", "Just started at my new job"). Signals for replacing an older vehicle could be looking up vehicle review pages on facebook (does edmunds.com have a facebook page?), like you mentioned.
So I don't think it's impossible (but definitely non-trivial). I have no idea if it's possible to target ads against the sorts of things people post to their wall/photo albums.
I don't think Facebook has the data yet to detect that you are about to buy a car. If you start liking car reviews, maybe, but that's not a typical behavior.
For other products (especially the ones you didn't know you needed), Facebook beats Google hands down. So don't pass judgment one way or another too quickly.