Facebook Toolkit for .NET Developers
Microsoft has just released a Facebook SDK that enables .NET developers to quickly and easily leverage the various features of the Facebook Platform. This toolkit has evolved over time with input from the community and from Microsoft. The latest release (v3.0) includes new architectural improvements and provides an asynchronous interface for using the toolkit from Silverlight and from WPF.
You can find more info on the SDK and detailed instructions about how to create a Fackbook application here.