The graph shows the growing referral coming from social media sites to the website Gawker Media. It tells the story of which social sharing buttons to place for easy clicking, if your website’s potential users looks like Gawker Media.
StumbleUpon has a big share here, but might be a skewed information, that is only particular to Gawker Media’s internet-first-movers-audience.
Facebook is the winner, which might reflect a general tendency: “After their launch of Open Graph in April this year, Facebook has grown from 15% to 35% of all social networking referrals.”
Source: WordPress Hacks