I'm with the "None" group, which is probably going to be a trend around here.
In general, I think the IM thing has moved to cell phone texting for most, since it's rather boring for most on the PC. The most common use I've seen is with sales people while at work, since they like to keep multiple channels open.
When it's used, it tends to be a clique based chioce, since the whole point is usually keeping in touch with some select group. This highly distorts the choice, since those starting the clique generally define the client used.
I used to have the MSN Messenger enabled for a while, but even then I never really used it because the couple people I knew using it weren't any more interested than I. We tended to want to reach a wider audience so we either used email or our forums. IM is too limiting to be useful, unless you don't have a phone available or often 'talk' internationaly.
Bitman