For example Uber gives you credit if you get friends to sign up. Many other companies use this tactic too. It seems a product could get explosive growth using a method like a pyramid scheme, but I'm not sure it's ethical...
Are there any other examples of ethical pyramid schemes in business?
This all applies when the pyramid scheme is zero sum (i.e. the transfer of money up the pyramid). I once had the idea for a company that would do tutoring in a pyramid structure where a tutor would be one level above their tutee in this pyramid and that there would be non zero-sum interactions since the tutor also cements their understanding by trying to teach someone one level below them, with some monetary commission involved too for a tutor's tutee's tutee's tutoring, etc.
That wouldn't be a pyramid scheme by the aforementioned definition though.
You can't really have an ethical pyramid scheme by the definition, I think people just confuse "pyramid-shaped" things with the precise definition of a pyramid scheme.