How do you release a platform that requires people to function?

You'd assume in order to maintain users and costumers, you'd need to actually provide your service. The service I want to release requires user-interaction on a relatively large scale.

How can I build up a user-base prior to release in order to ensure that upon release, there are actually users. I think the closest thing I can think of, which is still relatively distant to what I want is Ebay and Kijiji. If Kijiji or Ebay didn't exist today, how would it get off it's feet if there is nobody to use it yet?