ebucks is a different entity. You have different things you can do to earn ebucks. But your ebucks account is seperate to your FNB account.
I've had an ebucks account for ages, never banked with FNB. Whenever I buy through Kalahari i just quote my ebucks account number and get rewards. When I rack up enough ebucks I pay for something only using ebucks.
ebucks might be a subsidiary of FNB but it's not linked exclusively to it. If you dump your credit card you had with FNB etc then you can still use your ebucks account to claim rewards etc