I suggest SO to friends almost on a weekly basis — some of whom have joined, and really enjoy it. Would it be out of the question to consider referral-links, which award the referrer with a "Recruiter" badge after two or three friends have joined, and reached a reputation of 100+?
Perhaps these badges could be named "Recruiter" or "Scout."
Even if implemented - most recommendations for a site are going to cause the listener to just type the site name into their browser (either navigate or search). I doubt the referral link would be used in reality.
It also ties into the discussion about affiliate tags - if readers here treat them as spam, wouldn't they look at your link as spam?
But please keep recommending the sites ;-p
Indeed, this is now possible:
https://blog.stackoverflow.com/2010/09/announcer-booster-and-publicist-badges/
We have Announcer (bronze), Booster (silver), and Publicist (gold).