What's going on here?
Facebook isn't working for General Motors because their marketing strategy doesn't align with the platform. There's no logical connection between their off-line marketing and the on-line social network.
Ford, on the other hand, uses Facebook, among other things, to extend the conversation created by their "Swap Your Ride" campaign.
GM cutting Facebook ad dollars makes sense given that their strategy doesn't align well with the platform.
Just because Facebook is big, just because it's in the news, just because it works for Ford, doesn't mean it's right for GM.
That's the sign of a professional marketer. They let the strategy drive the tactics, not the other way around.