Variable Rewards Referral

Random rewards create a cognitive bias that increases our desire to take our chances. This is because the uncertainty of the reward releases more dopamine - the neurotransmitter that supercharges our desires - than the reward itself, as do gambling.

You can easily use the mechanism in your referral program. Instead of using predictable rewards (if you refer 1 people, you win 50$), create a system of variable rewards (if you refer 1 people, you can win from 10$ to 100$)