Well, AOL is the leader in IM clients: approximately 41 million home users in total (and approx 10 million work users), where MSN is in second place (last time I checked) at roughly 18 million.
The difference being AOL offers both on-line access to the internet as well as their intranet which outside surfers cannot access. As well as there are IM users like the rest of us who either use iChat or AOL IM without actually subcribing to AOL.
AOL's subscription rate is amazing. I used to work for a web company who were at one time Atlantic Canada's only America On-Line's Content developer. I even had a chance to visit AOL University in Virginia and had an opportunity to learn some crazy shiznit with their proprietary tools.
At the time, their client base was approximately 20 million subscribers... they practically double their audience every several years (give or take).
I doubt MSN will ever break into that level of audience, unless of course they reach a deal with AOL to allow sharing, which I doubt since AOL is a part of TimeWarner who also own Netscape... etc. etc. There is a little bit of bad blood since the days when MS used to co-develop AOL's internal browser.