Sharing is big on the Web. We all know that. But exactly how big? ShareThis has some answers in a study it put together with Starcom MediaVest Group and Rubinson Partners . Looking across the sharing and clicking habits of the more than 300 million people a month who pass links with a ShareThis button on over a million websites (producing 7 billion pageviews a month), a few things stood out. Overall, sharing now produces an estimated 10 percent of all Internet traffic and 31 percent of referral traffic to sites from search and social. Search is still about twice as big. When it comes to sharing on the Web, Facebook rules. Facebook accounts for 38 percent of all sharing referral traffic. Email and Twitter tied for second with 17 percent each.