If you have been on recently you have noticed the "Facebook" chat bar down on the bottom right of your screen. In order for you to chat to someone you have to follow them and they have to follow you. After you both are following each other go to their profile and click "add as friend" to the right of their name. Since you are both following each other it automatically adds both of you to each others chat. Now when you only want to chat to one person you can follow and add them. =]