I try to spread the word on Stack Overflow awesomeness whenever I get a chance. How about providing a "Recruiter badge" which you obtain when you've managed to get XX (10, 50 or 100) number of people to sign up and they reach a certain level of reputation like 50 or 100... something to indicate they’re active on the site?
This is basically a simple referral scheme: when you sign up, you can enter the ID of the SO user who was kind enough to tell you about the site.
UPDATE
To all the downvoters: don't we want to reward behaviour that benefits the community?