At little late with the news (but Michael's posting prompted me to follow up) - it looks like Facebook have made another one of their leaps to keep the other Social Networks wondering what they need to do to keep up.
Last year Facebook opened up their platform. It gave app developers the chance to write and deploy viral apps that any Facebook user could add to their profile and interact with. Sadly that has resulted in an increase in Facebook spam (hint: I don't want to battle your virtual vampire or send you a pixilated pina colada) but over time hopefully ranking of apps and increased user awareness (just like for any other sort of spam) we'll start to see some really useful apps turning up on the platform.
Essentially it means that as a site developer you now have access to the Facebook infrastructure to build your own niche social network - saving you time and effort, giving you a robust, scalable platform, and giving Facebook potential new users as people sign up to take advantage of your network.
TechCrunch call it a clever move. I'm keeping my eyes peeled for the first really cool use of this extension to the platform.