To calculate social media ROI, you first have to have a bunch of metrics that you have gathered throughout your social media marketing campaign. Make sure you have the correct metrics since it will highly impact the end result. When you have it all divided into each social network, you can finally start calculating your social media ROI.
Social Media ROI can be calculated in terms of interaction made over social media websites. Below is a nice presentation where the social media giants facebook, twitter's statistics are shown along with memes, youtube streams, infographics and blogs.


Social Media ROI Calculator