I predict that the same people will win this over and over again, because they already have the most referrals and if these referrals are active, then this user will WIN every time this contest is hosted. The referral side of this should actually be made more fair, by only rewarding wagers that were made by new referrals that were added from the start date of the competition.

This will cancel out the historic advantage that these users have and it will serve as a incentive for people to get new referrals for this competition.