I'm still trawling through the myriad volunteering sites currently out there, but it's taking a while to review them all. Meantime just thought I'd say I quite like what they're trying to do with Idealist.org, even if the site feels a little too complicated to engage with easily. In their own words "Idealist is an interactive site where people and organizations can exchange resources and ideas, locate opportunities and supporters, and take steps toward building a world where all people can lead free and dignified lives."
Unfortunately while it is a little more Web 2 than some of the other sites, it is still isn't quite the sort of social networking we're talking about with the Urban Survival Project, but if you're interested in social engagement it's worth a look...
Idealist.org
