Instant Messenger users are fanatically loyal to their own programs and their respective program fixation is based on obvious things like on what programs most of their friends are signed on and possibly what features they offer.

Another reason is simply the first program you ever used.

I distinctly remember the first time I used IM. It was the summer of '98 and I was back home from college for the summer holidays. A friend called me and asked me to download Yahoo Messenger and off we went. I also remember jumping up and down in joy for a considerable period of time, thrilled at the prospect of simula-talking with all my friends not paying ( especially then ) the autrocious long distance phone call rates. So it was Yahoo Messenger for me.

MSN Messenger and Yahoo seem to be the most popular in India. AOL definitely seems to be the most popular here in the US. They are also programs like Trillian which combine all your accounts in a single account.