Tap the Menu key.

Scroll to Installations and tap the Navigation key.

Scroll to Facebook and tap the Navigation key.

If it's the first time you use the application, you need to log on.

You can now use the application.
Please note: Facebook isn't part of the original application package and therefore only a general description is given here.
You have a number of options, such as:
Write status updates
Send messages
Write text on Wall
Upload pictures
Select application settings

Tap the Menu key to return to standby mode.