1) GM is exactly the right type of advertiser for Facebook. They don't want you to buy a car right now, they want to get the brand in front of you. They, with all their resources, can't get it to work.
2) Google Adwords-style advertising is the absolute shortest line between a person who has a need and someone willing to fulfil that. To be worth more, Facebook has to either make "brand awareness" advertising bigger than "need fulfilment", or somehow figure out a way to beat Google at need fulfilment before Google heads them off. The thing about need fulfilment is that the user is already motivated to solve their problem, at this exact point in time that they're searching. You just have to fulfil that. That's worth pure gold for a marketer. Currently on Facebook the "need" is to get more cows, or whatever. To be "worth more" than Google, you need to use the social graphy thing to figure out how to get someone to spend money. But see #4.
3) I haven't spent one cent on Facebook, so either find a way to get me to spend one cent, or increase the value of the "whales" somehow.
4) Google knows a lot more about me than FB does. I spend every day telling Google what I'm thinking about, via the searches I do. Facebook, I might click a "like", or post a status. You can be sure that the things I'll "like" are a tiny subset of the things I search about. Google therefore has to hold back in which information they use about me, so they don't look creepy. Zuck would be happy looking creepy, but he's not able to get enough info to even begin to look creepy. [edit: so really, Google could make more by flicking a few "add creepy" switches. If FB desensitises us to that first, Google is still more likely to be able to leverage it.]
So, what would make Facebook worth more? Maybe if they leveraged the asset of having-everyone-on-board in a way to make the specific applications (e.g. search) irrelevant. They've done a reasonable job already, what with integrating it into websites, but that still isn't making them money in any direct way. Indirect ways are not going to beat the direct way.
So, Facebook is still a bit like Google pre-Adwords. We have a great lock, still looking for the key.
[edit: please tell me why I'm wrong. Really - I'd love to hear a way Facebook could win, would be an awesome lesson!]