Bad news spreads twice as fast as good news. There was a great article on this in the good news recently about how people prefer bad news to good news.
Think about the last time you ate out at a restaurant where the food was good. Did you tell all your friends they must go eat there?
Probably not. If they asked what you did last night you might tell them it was nice. But you're not going to go out of your way to refer friends to the restaurant.
If you got food poisoning from eating at the same restaurant, I bet you'd tell a lot of people not to go there.
So if the restaurant wants to get its customers to refer them based on the strength of their service alone, they have to surpass your expectations so that you are so blown away that you feel compelled to recommend them to friends.
To spread good news you generally need to have exceeded expectations. So, identify the point at which that happens. Can it be engineered to happen more often? Post-checkout? Post-delivery? Post-service? Which brands/customers have the greatest likelihood of having their expectations exceeded?
That's where the magic happens.