Usually, games have a built in chat system, which the majority of gamers (especially here) would use. I, in fact, do not even use XFire. I actually am starting to use Google Talk more than Yahoo IM or AIM, so why not add a Google Talk box, eh?
Probably because I am not the majority - others may use one of the main-stream instant messengers.
Just remember, everyone has an extra profile field: Biography. Do with that as you wish (of course, any content has to abide by current rules)!