Living in Germany, I know no one from my circles who uses Payback. This is in my gut feeling also a central reason why such loyalty programs push/nudge so hard to get people as customers - because so few people want them.
The only time when such loyalty programs were tried out by quite some people in Germany was when in 2001 the Rabattgesetz [discount law; https://de.wikipedia.org/wiki/Rabattgesetz] was abolished. Then, for a short time, quite some people tried out the now upcoming loyalty programs, but soon stopped using them because quite some people were very vocal about telling the privacy implications of these loyalty programs.
"Will you pay cash or with credit card?"
"Why, cash of course!"
"Thanks. Do you have a Payback card?"
"Certainly."
> "Certainly."
Rather: "Thanks. Do you have a Payback card?" - the person holds a speech about the privacy implications of Payback when there is a long queue at the checkout. :-D