Sometimes I use IM, sometimes I just go back and forth between various web sites, sometimes I'm working on something for school (writing a paper, my art apprec. course is on-line), talking to a friend IRL, and I can get up and do any number of things around the house and just leave my computer on and come back to it. But sometimes I just check in for a few minutes when I check my email then get off-line.
That's exactly what I do as well. Also, all my music is in my computer, so it never shuts up.