Paid and dedicated
I can't find it now but someone recently characterized Duolingo as mercenary -(Although they didn't use that term) - Because they have paid employees. They do good work and should be paid well. I was paid well at the college where I worked and my colleagues were both well paid and extremely dedicated. It has been my good fortune to work with such people who have developed themselves to their extent and influenced many including me. I applaud Duolingo staff both paid and volunteered. Keep on learning.