Facebook.NET provides a .net library for use in developing Facebook applications and accessing Facebook APIs. The library primarily geared around and optimized for developing ASP.NET-based Web applications, both FBML and IFrame-based Facebook applications through an intuitive API and small set of server controls. It does support the use of the Facebook API from desktop applications as well, and will eventually enable Silverlight application usage. The library is built on .NET 2.0, but should run on future versions of the .NET framework as well. Applications using Facebook.NET can be written in either C# or VB.NET.
Go to http://www.codeplex.com/FacebookNET
See also these related projects.