It makes sense that people of certain countries would use whatever service is most wideley used. In the USA for instance, the most widely used IM client seems to be AIM, probably due to AOL's stranglehold of the ISP market in years-gone-by. When I go on AIM, I see most of my American friends online there. On MSN however, only those who have something like Gaim or Trillian are online, with only a scattering of those who actually use MSN. Here in Australia though, we predominatly use MSN Messenger. The major media network here sponsors MSN (ninemsn.com.au), and most teenage internet users here don't even know what AIM is. Skype seems widespread throughout the world, since it has a service that neither of the other clients have (yet), which is SkypeOut. Y!IM is almost dead, however it may kick back when Windows Live Messenger starts accepting Yahoo! Instant Messenger contacts. What is the current IM market share of the above three clients in your country?
I use windows messenger instead of msn because it has less bloated garbage. The live messenger was very nice, but it was bloated with useless shit so I uninstalled it. AIM ad-hack seems realible, and I have the most people with it.
In Singapore, the old rage was with ICQ a few years back; but MSN is becoming more popular by the day, perhaps because most people have Hotmail accounts that are already accessible to MSN.