Well, I know Ian used to run a ton of referral contests on Marapets in the past, but he hasn't done any since - but then again, why haven't pet game owners ran one now?
Like you could giveaway an expensive cash shop item, a limited edition pet, etc, anything is possible.
What do you guys think? Should some of the major pet games run a referral contest? Its a good way to get people talking about your pet site on the web - remember, publicity is publicity.