Facebook has done something that further represents some of its goals with its open platform--allowed non-Facebook users to join Facebook with third-party apps automatically added to their accounts. This is in an effort to work with developers that have built applications for services that already had a good amount of users. A pre-existing community, if you will. And in order to better incorporate the members of these pre-existing communities, Facebook now offers special registration links for non-Facebook users to take advantage of the Facebook apps.
What's good for developers and site owners is that there's now a more seamless manner in which their applications can work in conjunction with their existing services, and push for the entire community to stay intact. This should also be encouraging for those that have developed Facebook applications in hopes of building out a stand-alone service. The Facebook app can be used to help gain more outside users, to a certain extent.