If you are still wondering why Google is pushing so hard with its new product Buzz, it is because it wants in on social traffic. For many sites on the Web, social traffic coming through Facebook, Twitter, and MySpace is beginning to rival, and in some cases overtake, search traffic as the single biggest source of traffic. This traffic comes from shared links, photos, and videos. By its own numbers, 5 billion pieces of content are shared on Facebook every month.
What isn’t easily appreciated is the extent to which such social sharing is tied to different identity and authentication platforms across the Web. If you can log into a site easily using your Facebook or Twitter account, it is easier to broadcast links from that site to your friends.

To get a sense of which services on the Web drive the most sharing, I asked Gigya for some stats. Gigya powers sharing widgets on more than 5,000 content sites, including ABC.com. NBA.com, PGA.com, Answers.com, and Reuters. Consumers can click a share button on these sites and send an article link, photo, or video via a menu of different services including Facebook, Twitter, MySpace, Yahoo Mail, Gmail, and AOL. Over the past 30 days, people have shared almost a million items over the Gigya network. Facebook and Twitter dominate with about three quarters of all shared items between them. Here is how the services break down (note that these are relative numbers) :
Distribution of shared items
Facebook: 44%
Twitter: 29%
Yahoo:18%
MySpace:9%
It makes sense, people prefer to broadcast links rather than share them one at a time via email. Although Yahoo makes a strong third-place showing. When it comes to authentication, simply using your existing username and password to log into another site, Facebook is still the most popular via Facebook Connect, but only just barely. Google via Gmail and Yahoo are almost equally popular, at least on certain types of sites where people are just reading for themselves like news sites. On entertainment sites where people are more likely to share content, Facebook Connect makes up the majority of logins.
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