Exchange markets & trading platforms
Whenever a new person enters the Bitcoin ecosystem, one of his/her first questions is "How to buy Bitcoin?".
There are many different ways, some even anonymous. But if your goal is to make money you should be referring exchange markets and trading platforms that have an affiliate program to those people.
Most of the online services offer some kind of economic incentives for existing users to lure newbies. Usually, in the form of a commission from paid fees.
Every time your referrals buy/sell bitcoins, they pay small trading fees from which a commission is rewarded to your account.
Because people usually sign up these services with an intention to spend money, it's an easy way to build decent passive incomes.
As an example incomes from Bitcoin affiliate networks.