I privilege Emerson and Bugg-Levine's definition. After all, these are the guys who coined the terms "blended value" and "impact investing."

"Impact investors intend to create positive impact alongside various levels of financial return, both managing and measuring the blended value they create."

So the positive social/environmental impact has to be intentional, managed, and measurable—which would exclude most social media. They also believe that additionality (investing in organizations that wouldn't receive money from traditional angels/VCs) is an essential component of impact investing.

See http://impactentrepreneurs.wordp ress.com/2012/04/16/an-introduct ion-to-impact-investing/ for a diagram of the investment spectrum and more around definitions.