is credit card referral bonuses taxable income ?

That’s a bit tricky, but I would probably say yes.

Cashback bonus is, or the cash value of points or other rewards that are earned when you use the card or not taxable income. That represents a discount or rebate of the merchant fee that the bank charges to the business and the business passes on to its customers,  and a discount in the price of a service does not create taxable income.

However, a new cardmember sign up bonus is taxable income, because it does not represent the same kind of ongoing discount of user fees but is instead a lump sum of cash, or cash value.

given those two situations, which are clearly established in tax law, I would say that a referral bonus is more like the sign-up bonus and less like cash back that is based on your purchases.  So I would conclude that a referral bonus is taxable income. However, you will probably find that the credit card company does not issue a 1099 unless you are paid more than $600 in a year.