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Friend Connect is a tool which enables any website owner to add some code to their site and get a number of social features. You know, all that stuff you usually can't be bothered to install plugins for: user registration, invites, members gallery, reviews, message posting, and - most importantly - third party OpenSocial apps.
In practice, this means that anyone will be able to log in, for example, with their OpenID on some blog, and converse with their Gtalk, Facbeook, or Plaxo friends. The web as a platform, it's finally happening, folks.