Your impression of Facebook's user base is about five years out of date. Facebook is closing in on having ~1,000,000,000 members. I would suspect millions of those members are looking for a new car right this second. Facebook needs to find better a way, like Google did, of matching those members with advertisers.

But are they looking for a car while they're on Facebook?

This has no meaning if they see what they are looking for right in front of them.

Probably. I see people browsing/completing their tasks while having Facebook open in another tab pretty much all the time.

Most people are on Facebook, so there are a large number of people looking for new cars. Facebook's big pitch to advertisers is that it can target better than anyone else. Why can't they find people who are looking for a new car?

Why do you say that? Granted I don't hang out with starving students, but the people I know on Facebook are quite capable of buying new cars. I don't know why you think moms aren't interested in GM cars. Around here GM trucks and cars are selling pretty well.

In any case a lot of people here, as another poster said, are confusing direct advertising with branding. I'm not in the market for a new car, but if I'm on Facebook, it's more than likely because I'm bored. If a car ad that looks good is on the page I'm on, I'm likely to click on it and look at the car. A year or two from now when I might be in the market, that make of car I keep seeing on FB is likely to be on my mind as a possible candidate.

And now that I think of it, I keep seeing brand new GM Yukons on the road and I like the way they look and they look like they are good tow vehicles. If I saw a Yukon ad on facebook, odds are I will click through.

I think a better question is how GM measures the effectiveness of their campaigns on Facebook. Its not like you can track the conversion rate from ad click to car buy.

I'd imagine that they're doing a form of A/B testing geographically. All other things equal, its easy enough to only do an ad campaign within a certain radius of a dealership and measure the change in that dealership to others in the state.

You can't track the conversion rate from a TV ad either. That doesn't mean you can't notice a positive/negative effect related to the presence/absence of the ad. Especially at the scale of GM.