Google Analytics has a number of features that allow you to measure the impact of social media on your business: Social Activities, Social Referrals, and Social Interactions.
What To Use
Social Activities
A social activity represents a social action (e.g. +1, comment, vote,
share, etc.) that happens off-site on a social network and references
content on your website. Google Analytics automatically enables measurement
of social activities related to your content for a variety of social
networks.
Learn more about measuring Social Activities.
Social Interactions
User interactions with social buttons embedded on your property, such as
the Google+ "+1" button, or the Facebook "Like" button, represent social
interactions with your content. These social interactions are
valuable signals of user engagement and can be measured using Google
Analytics.
Learn more about Social Interactions.