AIM (that is, Adium) is always on on my computer. I have nearly 200 contacts in my list, most of whom I never talk to, but just like to check there profiles, or some are friends back home who I keep on there basically so I don't lose their screen name. As I add more people to the list, I am constantly having to prune names that people no longer use. If I'm using a computer somewhere that doesn't have an AIM client, I use AIM express. I have 5 or 6 screen names, so my 3 names are still signed on on my computer even while I'm chatting from another. As a college student, it's a great way to keep in touch with family back home, friends at other schools, etc. Also, all of my friends at school use AIM, and are always signed on. Even if someone just has an away message up, it's a good way to find out where they are. AIM is crucial to my day to day interaction with friends and family. It let's me talk to my girlfriend who is 5 hours away without using daytime cell minutes, or while I'm at work, waiting around for something to do. I suppose you could say I'm addicted to AIM, but it's really more subtle than that.
There was a quiz a while back that told you how "addicted to AIM" you were. I haven't taken it in a while.
Edit:
Found the quiz. There is also a Are you
severely addicted to AIM? quiz there.