Google and Facebook on Thursday announced the availability of competing authentication systems that enable Internet users to sign in to third-party Web sites using either their Facebook or Google Account login details.

Google Friend Connect lets users sign in to participating Web sites through their Google, Yahoo, AIM, or OpenID accounts. Participation in this case requires site owners to supply Google with some information and to paste the code for desired OpenSocial gadgets into their Web pages. After that, site visitors can log in using the Google Friend Connect widget.

Google Friend Connect is an application under OpenSocial, a set of APIs for building online social applications. It was made available as a preview release in May to sites that Google approved. It's now available to any Web site that chooses to implement it.

"Friend Connect's goal is to facilitate an open social Web," said Google product manager Mussie Shore in a blog post. "Using open standards like OpenID and OAuth, Friend Connect makes it simple for people to instantly interact with one another on the sites that they already love to visit. Additionally, Web sites that use Friend Connect become OpenSocial containers, capable of running applications created by the OpenSocial developer community."

Facebook Connect represents a similar but more expansive system that allows third-party Web sites to accept Facebook login information. It also provides users with the opportunity to share content created at a third-party Web site on Facebook.

This may become more meaningful to third-party sites when Facebook's monetization options mature. But even absent monetary rewards, the promise of exposure to Facebook's large audience could spur publishers to choose Facebook's system over Google's.

Facebook Connect promises something that Google Friend Connect doesn't: true identity. Facebook says its users use their real names on Facebook, a claim that while not universally true could nonetheless represent a real competitive advantage against Google. Social Web sites looking for civil interaction among users may opt for Facebook Connect over Google Friend Connect out of the belief that their users will behave better if they can be easily identified.