This article does not argue a mathematical ROI exactly but it is a very interesting view of why organisations do social media marketing.
One point that isn't made here is that this is not really a matter of an A/B choice. A good content marketing strategy will not only get you plenty of shares, if it is organised, it should drive excellent SEO as well.
83 percent of CMOs claim that SOCIAL SHARING is the primary benefit of social media marketing. Wait. What? You mean it’s not clicks, Likes or new followers? It’s not even TRAFFIC for heaven’s sakes? The CMOs in this study are smart people because they know that 70 percent of their customers are more likely to buy something when they see content about a product or service shared by a friend.