In the Linode model, the user must keep their Linode instance for at least 90 days to get the $20 credit. Since the cheapest Linode plan is $20/mo, that means

In the Linode model, the user must keep their Linode instance for at least 90 days to get the $20 credit. Since the cheapest Linode plan is $20/mo, that means Linode makes at least $40 per referral. Even if that was a 2-sided incentive (the referred user gets a free month too), Linode makes at least $20 per referral (minus costs, so you have to watch your margins).