We are up to 27 people now, so for us it's super important to have everything scheduled and organized. We have shifts and a shift switch board, that let's us switch shifts when people are going on vacation, or have other items in their schedule.

I know Wufoo started off pretty informal, and their formalized support rotation was largely in reaction to not everyone pulling their weight. In my experience it's really important to formalize things so that everyone is pitching in, and it's not just up to a few people to volunteer.