You can access the Facebook Promotions Guidelines at: http://www.facebook.com/promotions_guidelines.php
In short, Facebook will not allow incentive for Sharing, but will allow incentive for "round-trip invitations" - that means that if I invite you and you also signup, then I get additional incentive that the contest sponsor defines. This type of sweepstakes is what we call a viral sweepstakes. We do offer this type of sweepstakes as a premium offering. If you want to have this option you can call us at 1.800.519.1850 or fill out our contact form and we will work with you to develop a premium solution that fits your needs for an additional fee.