This information applies to Nokia N900 starting from sw. version 10.2010.19-1 (PR1.2).
First you need to have Facebook account created and Facebook chat account activated. To check you Facebook chat user name and for more information about Facebook chat please visit http://www.facebook.com/sitetour/chat.php. To check your Facebook chat user name, log in and select Other - Windows/Mac/Linux. Pop up window opens where your Facebook chat user name is mentioned. Note. To N900 Facebook chat user name needs to be entered in same format as Jabber ID is shown in that pop up window.On your N900 tap the Application menu icon and select Settings > VoIP and IM accounts > New > Facebook. Add your User name and Password (no need to change other settings) and select Sign in (now your account is saved). To change your availability tap the status area, choose your status and select Save.