All it takes is one of your family members to download some dodgy app and give it access to their contacts for the secret number to be revealed. I suspect this

All it takes is one of your family members to download some dodgy app and give it access to their contacts for the secret number to be revealed. I suspect this is more likely than eg Facebook leaking a number by mistake. You probably have gotten lucky by sharing your secret number only with a small number of people though I don’t know for sure.