by Rafe Needleman
July 23, 2008 3:59 PM PDT

Mark Zuckerberg today officially rolled out Facebook Connect, a way for apps not on the Facebook Platform to leverage the Facebook social network. It's an extremely powerful idea, and the demos we saw at the F8 conference were much more impressive than the MySpace Data Availability project that rolled out yesterday.

Facebook Connect allows other Web sites and apps to have their users log in, or authenticate, to the Facebook system, and once logged in, their social network comes with them. The reason it's a bigger deal than Data Availability is that it's two-way. Not only does your Facebook data come to you on the external site, but things you do on the site can be reflected back to your Facebook profile and news feed. Two examples:

Digg: Users on Digg will be able to connect their accounts to the Facebook profiles, and then when they digg a new item, that information will get fed to their Facebook feed, and presumably will be seen by all their Facebook friends. This is one of the best examples of an app exercising the "virtuous circle" of content and community that Zuckerberg discussed in his keynote today.

Movable Type: Facebook Connect will allow commenters on MovableType blogs to log in via Facebook authentication. Comments they leave will get posted to their news feed, which is somewhat cool. What I really like is that when a user posts a comment on a blog, they'll see which of the other people commenting on the item are in their social network.