When you want to increase the number of friends in your list* - let's say from your Google Talk account, you have more than one possibility:
You will then be prompted to input the details of the username you're adding to the list - of which only the type of your Account that you're adding the friend to and the respective Username are mandatory:
You need to specify the domain when writing the username of the friend (e.g. email@example.com).
This step is necessary for making sure that the servers used in the communication process both support jabber protocol. For example, yahoo doesn't.
This means that if I am logged in my Google Talk account, I can add friends with accounts at jabber.org, gmail.com, or most of jabber-supporting servers.
Facebook*, on the other hand, although uses the same protocol, doesn't allow you (at least for now) to organize the buddies list.
Any regrets? Remove the name or go back to managing the buddies list.
Screenshots are from YateClient version 4.0.