We can't requiere the user to share or post the promotion on their wall, because we would be breaking Facebook applications' guidelines, which indicates that you can't require a user to share.
In particular, on clause IV. 4.1. Facebook developers privacy reads:
You won't encourage users to use (or prevent the access to content if they don't use them) the Facebook social channels, nor suggest that the incentive is directly related to the use of our channels.
However, since we can't require the user to share the promotion on their wall, in Easypromos we have developed a type of contest that allows you to select the winners of the promotion depending on the number of users they bring to the promotion and make them enter. This is the type of contest called "Recruiting contest" and it's available with Premium and White Label versions. If you want to learn more about this type of promotion, we explain in more detail how it works on the following link.