There is now an easy way to integrate Facebook on N900 conversations without installing Facebook Chat plugin from Extras-Devels repository. Thanks to Nokia Blog for the instructions.
Step 1. On your Nokia N900, go to Settings – VoIP and IM accounts and click the New button. Select Jabber as the service.
Step 2. Find your Facebook Chat username on this link by clicking on one of the clients. You just need to know the Jabber ID and your Facebook password.
Step 3: Just click on Others on the “connect your client” and there you will find your proper facebook chat ID. Password to be supplied is similar to your facebook password.
Step 4: Sign-in to your Facebook chat and there it is – Facebook Chat on your Conversations.
Use the same method if you want to integrate FB chat on Adium (on Ubuntu), iChat (on Mac) or Pidgin.