Microsoft and Facebook launched a partnership to implement a Facebook Development Kit so developers can start creating small applications and "widgets" for Facebook, which is a "social utility that connects you with the people around you". Facebook is one of the most popular network communities today, competing with MySpace and Hi5.
The Facebook Development Kit allows you to use Microsoft Popfly and Visual Studio Express to develop applications and enhancements for the Facebook utility.
Here are some links to getting started with:
Facebook site: http://www.facebook.com/
Microsoft Article: http://msdn.microsoft.com/vstudio/express/showcase/
FaceBook SDK Getting Started Page: http://developers.facebook.com/
By the way, Facebook has a 29 million users database....imagine the exposure of your applications!!!