Arienna Foley at Get Real has a good post about the social networking qualities of Flickr. I think she gets at why Flickr draws people in and creates community more than any other photo site.

Again I say, I think Flickr offers to the ACS a cheap opportunity to explore the power of community building and of pushing an event out into social space. If one volunteer at each Relay For Life were to take pictures during their event and post them to Flickr labeled with a tag we can create like "RELAY FOR LIFE" we’d have hundreds of pictures out there and the volunteers could link themselves into a nationwide community through Flickr.