Microsoft just released a Facebo ok client library to help make it easier for everyone to create some interesting applications. The SDK is broken into a few core DLLs, each serving a core purpose. You can download the SDK from Download the SDK.
- Facebook.dll: This is the main assembly that will be used by all applications. This has all the logic to handle communication with the Facebook application. This assembly also has specific support of XAML applications (Silverlight and WPF) to enhance the Facebook platform to make databinding and data caching easier.
- Facebook.Silverlight.dll: This is the Silverlight version of the main assembly that will be used by all Silverlight applications. This has all the logic to handle communication with the Facebook application. This assembly also has specific support of XAML applications to enhance the Facebook platform to make databinding and data caching easier. The REST API in this assembly is Asynchronous only.
- Facebook.Web.dll: This assembly should be used by Canvas applications. The main functionality supported in this assembly is to encapsulate the handshake between the Facebook application and a canvas application (both FBML and IFrame)
- Facebook.Web.Mvc.dll: Provide a support building canvas applications using ASP.NET MVC. Separated from Facebook.Web.dll to avoid all developers from needing to install the MVC bits.
- Facebook.Winforms.dll: This assembly provides support for writing Facebook applications using Winform technology. This provides a Component that wraps the API to make it easier to use from Winforms. This also contains some user controls to help display Facebook data easily.
If you want some helpful starting points, the Facebook Developer Wiki has some help too.