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.
White Star Line
www.white-star.org
All these networking sites - that aren't used
Feh.