An ongoing criticism of social media marketing is that it offers an uncertain ROI. However, measuring ROI has always been a central challenge for marketers. Whether you work in marketing or not, you have probably heard some variation of the often quoted adage, "Only half of marketing works. The problem is, we don't know which half." Identifying which marketing efforts are the most effective has long been a significant hurdle for marketers, inhibiting our ability to optimize (or justify) our efforts.