Muller, M. J., Freyne, J., Dugan, C., Millen, D. R., & Thom-Santelli, J. (2009). Return On Contribution (ROC): A Metric for Enterprise Social Software. ECSCW 2009, 143–150.
To get back ROI from social media, it is rather a difficult task perform. It is not very accurate. However, there is an article published by SAS Inc. USA that you may want to refer to. I dont remember the actual title. But the article title is about ROI from social media. If you cant get it, I will try to look for the article for you.
I like the article of Hoffman & Fodor (2010) as a basic background to the topic http://sloanreview.mit.edu/article/can-you-measure-the-roi-of-your-social-media-marketing/
The direct way is to ensure that all social media efforts are tagged the same way was your other campaign assets - then you can measure the campaign ROI using whatever marketing attribution model you use for everything else (ex. 1st touch, last touch, assisted conversions).
Then you have to approach it from a media value standpoint - the same way a media buyer would - reach & impressions, engagement, etc.
And then approach it like a PR professional and do a brand impact study to quantify the branding and brand reputation value.
The important thing is to make sure that you have visibility into as much data you can muster for of the key data points.
One thing that I have been a firm believer of is that social is a lot like celery when it comes to ROI. Celery has negative calories, and social media costs, in relationship to other forms of media, are so low, it basically makes your entire marketing mix cost per impression, lead, etc. go down.
That's the real value of social - it's like a good quarterback in football - it makes everyone around (all marketing activity) - more effective.