Facebook relies on your facebook community. It might be useful for a student with lots of friends at the same campus, or in similar circumstances. Probably more useful as a social-carpooling tool: find a friend to a go to a festival with, and then decide which weekend and which festival. Without a big facebook community I can't even test it.
SpaceShare builds customized ridesharing sites for large events, often with other travel-sharing: coordinating room, flight and taxi connections. Of all the event carpool sites, they tend to cost the most and do the most hand-holding, customization and outreach assistance; often they have a community-building focus beyond travel sharing. SpaceShare focuses on conferences and festivals, with a mix of professional, music and activist events. SpaceShare is prototyping carpool systems for repeating "events." All their events are free for the carpooler to use: find upcoming public events here. They're also capable of building church, school or business systems, again integrated with forums or other community-building beyond plain carpooling.