One of the issues I had with IRC was that, not anyone get notifications when a message is posted(I'm sure that didn't make a lot of sense, sorry)
So, unless you see the screen, you don't know what happen (you can get notified on some IRC client, but that is not activated by default), so, the "Instant messenger" isn't really instant.
I use skype, but since I use it in a "private way" (that sounded so much pervert, sorry about that), I don't really send it to that much people
And I don't like steam a lot. But It could be great if we found something convenient for most of all.