In the week leading up to the Facebook IPO, news broke that General Motors was pulling $10 million in display advertising from the world’s largest social network due to poor performance. But after digging into the story a little deeper, one thing became clear: There seemed to be an issue with how GM was measuring its display advertising campaign performance. Several news articles reported that GM was relying on click-through rates (CTRs) or last click to measure display campaign performance on Facebook. While the CTR is a great indicator of paid search performance, the online marketing industry has long known it’s an unreliable and ineffective way to measure the true impact of display. To get a clearer picture of why this is the…