If one, or more, of your contacts has multiple accounts with different messaging services, you can combine these contacts into a single one.
The resulting contact is called a meta-contact: a contact composed from different single contacts.
If you have a Jane Smith contact that is using three different messaging services like:
janes@facebook
jane.smith@gmail
jane_smith@hotmail
You can combine these contacts into a single Jane Smith one.
From the Contact List window, right-click one of the contact that has different accounts, and select Link Contacts....
From the left pane in the Link Contacts window, select the contacts you want to combine.
Click Link.
When a meta-contact has been created, the default contact that will be used to have a conversation with when you double-click on it, is the contact with the highest presence on-line.
From the Contact List, right-click the contact to separate, and select Link Contacts....
Click Unlink.