Microsoft has released Facebook for Windows Mobile. The application provides the following features:
Send messages to any of the people in your Friends list.
Take pictures and videos on your phone, then upload them right to Facebook.
Send messages or call people in your Friends list.
Manage your profile and post anytime, anywhere.
MobilityMinded has a very detailed step-by-step guide to the applications with lots of screenshots:
Image Credit: MobilityMinded.com