The site is like Amazon’s Mechanical Turk: you can sign up to do some fairly mechanical task that some organization has advertised. Unlike the Turk, though, you don’t get paid, because you’re doing it for a non-profit like the Library of Congress or Smithsonian.
The Extraordinaries only takes non-profits as their clients, but it does charge them. (Or it will charge them in the future. I’m not sure of the current status.) If that makes the service sustainable, then why not?