When online consumers use social sign-ins to share content from websites, they are increasingly signing in with Facebook. Janrain reports that for the first time ever, the social giant eclipsed Google as the most popular sign-in choice during Q1 2011, and it is also the most popular network for content sharing.

The social IDs and networks site visitors use to sign into websites are an indication of how much influence individual networks hold and how pervasive they are on the web, says Janrain. With this in mind, marketers might take Facebook’s growth as a social login for content sharing purposes to be yet another sign that the site is a marketing must.

Janrain reports that more than one-third of online users (35 percent) chose Facebook as their login platform, demonstrating nice growth over Q4 2010. Meanwhile, Google’s logins tapped out at 31 percent. This will likely upset Google CEO Larry Page, who is trying to get more searchers to sign in with Google for social purposes (such as using +1). Facebook is the No.1 network for social sharing.

Although Facebook is the leading login site, marketers should consider that consumers use a number of social platforms for sharing purposes. The study found that Facebook is the No. 1 network for social sharing, cited by 58 percent of consumers. Still, Twitter is a top sharing platform preferred by 32 percent of consumers, and 13 percent say they go to LinkedIn to share content.

With this in mind, marketers should be sure to monitor brand mentions across social media to see which platforms drive the most engagement for their particular brands and determine their best social traffic sources. As Brafton reported earlier today, 60 percent of content shared across social media includes links to external sites.