From Study Gives Insight Into Content Discovery Trends Across the Web’s Leading Publishers @ blog.outbrain.com:
Currently, search methods [...] send the largest slice of referral traffic to content. Links from publisher sites make up 31% of referral traffic to content pages [...], portal homepages (AOL.com, Yahoo.com, MSN.com) account for 17% of traffic, and finally, social media sites (Facebook, Twitter, StumbleUpon, Fark.com, reddit, Digg) send 11% of traffic to content pages.
What kind of content do people share online?
Download a high-res PDF of the report: Content Discovery and Engagement Report, Q1 2011 (.pdf)