There are a few people within the social media arena, specifically where related to sports participation. Amy Martin, from Digital Royalty, is one that I think talks some sense. Here's a short video in which she talks about how to measure your Social Media performance.
That performance is called ROI (or Return on Influence). This is an interesting concept for those who usually refer to ROI as Return on Investment, but fundamentally it is the same thing. You have invested in your social media strategy and plan, you invest in the actions that are integral to that plan and the expenses that come with it. You want to know and you should know how that investment is performing against your goals and objectives.
If you don't measure it from the start, how can you ever improve it?