I've done some research work in the past regarding online communities. All sorts of data, theories, stats, etc. See "The Well" by Katie Hafner. But one definition that seemed to resonate with me is this: "You're part of a community when your absence is noted....".
I am noting Erin's absence. She's a great rider who made great posting contributions. Haven't seen anything in a long while. Hope we haven't lost her....
