Hear me out on this. I'm not convinced it is necessary for users to pay in most cases. The only reason to pay for a service is if there is no free, privacy respecting alternative.
For example, DuckDuckGo is a reasonable search alternative to Google and Bing with no payment required and no privacy invasion. DDG is driven by contextual advertising. Lots of other services could be built around the same model.
In my mind, Facebook is the #1 example of a service that would be difficult to sustain for free without privacy invasion only because it's very nature is so personal.