Why Wall Street unfriended Facebook

In May, it looked like a $10-million slap in the face. This week, it began to resemble a $50-billion body blow.

When General Motors Co. said three months ago that it was pulling its paid ads from Facebook because it didn’t believe advertising on the site was effective, the move cast a sharp shadow over the company’s initial public offering. Two days later, Facebook‘s stock began trading – and then it began sinking.