Facebook made 85% of its $3.7 billion in revenue last year from ad boxes. Columnist Ben Kunz argues that GM's decision to retreat was not a mistake, any more than advertising on Facebook is a waste. "No free market spends billions on a media format with zero return," he writes. "What GM’s retreat really shows is the harsh reality that other brands must face: Making social-media communications work requires heavier lift than many organizations can muster."