I enjoyed instant messaging. It's not as immediate as a phone call - you need to wait for the person to respond. However, it does allow you to do other things while you wait. It's good for times when you don't have access to a phone. You don't get a busy signal.
The library can use it when you want an answer, but don't want to interrupt someone, when a reference question is being asked.
I like the idea of a Meebo too.