I know what you're thinking. Not another social network. I hear you, I'm in the same boat. It's hard to stay dedicated to more than one or two social network sites. But here's a new one that you don't need to be really dedicated to for the rest of your life, and it can come in handy when planning a trip.
Groople's (not associated with Google) GroopVine is a site where you and your friends can plan out and discuss a trip you are going on. You can create your own GroopVine URL address and book travel, schedule events and also import reservations you've made elsewhere. And if you see another GroopVine user with a cool trip, you can copy their plans into your trip. There are also interactive features like polls and videos.
Seems like a good tool for planning and for getting the group excited about the upcoming vacation. And although the site seems to be targeting the 20/30-something group trip, it also seems like it would be fun for families to use.