Several things have contributed to the success of Pinboard's pricing model.
Two of them have to do with the model itself.
- Because it's a one-time fee, there's no ongoing financial relationship. It's more like buying an app that way. (The pros and cons of app pricing economics have been thoroughly analysed to death by others.)
- Because the one-time fee increases marginally with each new user, people have a psychological incentive to sign up sooner.