Good idea - could also spread the wealth instead of 100% for the winner, 50% for 1st, 30 for 2nd 20 for 3rd. Or even 'referral bonuses tripled for 30 days' or something like that.
Maybe read this before doing that. It refers to link spam here on SN, but some comments are quite relevant to the amount of "Sat farmers" your campaign may bring: