From the comments:

> Stack Overflow will make both Spolsky and Atwood enormously wealthy, based upon the volunteer efforts of others. Who will get nothing.

The others won't get nothing. They'll get whatever rewards go along with participating in a forum: the opportunity to demonstrate prowess and be recognized for it, the chance to have specific questions answered in a peer-reviewed medium, and so on. Don't discount the value that people place in these things, even if they aren't being compensated financially.