I second the Facebook app idea.

And I really like the calendar that shows all my friends and what they are doing today. That would be really nice in Facebook and at this point I don't know if that's enough to not just sign up for another service, but also check it and get friends using it.

I can see someone who just wants to login, see what's going on and if there's something s/he can join and then log off and go. And I don't think being part of FB hinders that. It probably helps.

time travel!

This might be a good concept to run on Digg / Reddit submissions - the concept needs a fair amount of tweaking to match that application.