Microsoft released a Fishbowl which is a Silverlight based Facebook desktop client.
This is a cool application which will help you to use the facebook more easily. Brag and drop photos will upload the photos to Facebook and share.
Mini-mode offers an unobtrusive way to stay connected.
The full mode
Operating system: Windows XP with Service Pack 2 (32-bit edition only), Windows Vista (32-bit or 64-bit editions), Windows 7 (32-bit or 64-bit editions), or Windows Server 2008.
Processor: 1 GHz or higher for Windows 7 or Windows Vista; 800 MHz or higher for Windows XP.
Memory: 1 GB of RAM for Windows 7 or Windows Vista; 512 MB for Windows XP.
Internet connection: Internet functionality requires dial-up or broadband Internet access (provided separately). Local or long-distance charges may apply.
Graphics or video card: A video card that supports DirectX 9.0 or higher, and Shader Model 2.0 or higher.
Read this too from Microsoft
Fishbowl is a trial application and a sample demonstrating a unique user experience with Facebook content, optimized for Windows 7. It is available for download by technology enthusiasts and Facebook power users. Since it's currently a trial version, we don't support it. But don't let that stop you from kicking the tires (and from sending us your feedback)!
You can download the source code of this application from here
You can download the Facebook SDK for .NET developers from here