Your most successful customer is the one that no longer needs your platform, so a monthly fee or relying on ads does not work. You end up with a perverse incentive to make sure users only ever get "okay" matches, and never GOOD or even GREAT matches.
But if you charge a one-time fee, then the barrier of entry is too high and you won't get many people buying, especially once the people who are frankly undateable start bad-mouthing your app.
You could try a method where you only pay once you decide you landed a good match, but that's going to be impossible to enforce without greatly giving up privacy.
