There are lots of different ways to measure social media ROI (return on investment). Some are short term (increase in web traffic, mentions, leads, etc.), but many are longer term and thus much harder to measure. Whenever you measure anything, you want to be very clear about exactly what it is you’re measuring and what it means.
This video, Social Media ROI: Socialnomics is by Erik Qualman. He’ll give you some ideas of the longer term benefits of social media.