The vCard module adds a vCard download link to every users profile. This link makes it easy to add users on your Drupal site to your local address book. Since we love microformats, there is also a theme function (theme_vcard) for displaying user data in the hCard format.
After watching Chris Messina's existential DiSo Interview, I decided to go check out a bit more about the distributed social networking stuff Chris and Steve Ivy have been working on - there are 160+ people on the mailing list now, and hundreds of threads.