You can now create a new way to grasp how someone actually comes to your website. Although still in beta you should be applying for this great new Google Analytics Multi-Channel Funnels Tool. As a marketer with Google Analytics on their site to measure the impact of all the channel interactions a visitor has with the site leading up to a conversion or purchase... now you can also tell what social media they have been using.
http://www.google.com/analytics/analytics-funnels.html
Read more at:
http://searchenginewatch.com/article/2072108/A-Step-Closer-to-Social-Media-ROI-with-Google-Analytics-Multi-Channel-Funnels