I used ICQ for a very long time. My ICQ number is quite low compared to what most people have. It only has 6 digits (

). So, needless to say, I have built a whole lot of contacts over the years. Almost all my online friends use ICQ.
I don't prefer ICQ, it's just what the people I know use.
I've tried MSN, and the interface is sleek. Neat. I like it. It's a valid choice. And I do know, as I said before, a couple of people who use MSN (ironically though, they are Mac users, go figure

). At some point, I even had a friend (only one) who used Yahoo. So, that's when I decided to try Trillian. I certainly didn't want to have to keep 3 messengers running.
And when I run Linux, I use Gaim, which can also connect to all networks. Only problem with it is that ot doesn't do file transfer. Bummer. It'll come.