I think your choice of im networks depends exclusively on who you want to talk to and what they use.

I used to use icq heavily, when all my friends did. Now my friends either use aim, or just email or call, so ICQ took a pretty long hiatus. But now people at my job use icq, so I had to dust off the old number and find a client for it.

For aim, I use a slightly older aim client with a slightly older deadaim wrapper, and the ad server pointed at my local webserver (just to make things particularly quick loading). ICQ I use the open source client Miranda IM. I don't use MSN or yahoo, because I don't like the fact that MSN refuses to listen to what you tell it to do, and yahoo im is just ghey.

A lot of my friends who are on multiple services use trillian. In linux I used to use gaim, but now I'm usually too busy when I'm in a linux desktop to worry about im. on my mac laptop, I use ichat and say again "screw icq" because usually on my laptop means at work, where it's easier for the people who talk to me to just walk the 20 feet to talk to me than to type at me and have me ignore them.

But yeah, it all depends pretty much on what your friends use, ya know?