Back when I didn't know who was going to come out on top and become the dominant system, I got accounts on upcoming.org, evite.com. freindster, facebook & myspace.

Near as I can tell only the last one of those gets used. I can't find anybody I know on the other ones, except Evite, and they haven't used that in about a year.

Google calendar looked promisuing but nobody seems to update that anymore either. Seems to me like most of this is still being done via regular old email lists.