A reasonable concern
I get the feeling sometimes that DuoLingo, which is now free and has been advertised as free, is going to begin to grow rapidly. When it grows very big I get the feeling that the developers are going to charge for it, like many other small sites on the web have done when they increase in popularity. If DuoLingo becomes commercial, I will stop using it immediately, not because the developers do not deserve money for it, but because it was advertised as free and they can get money in other ways for DuoLingo. I would like to know other people's opinions on this.
They could probably earn some funding by introducing a scheme for schools and colleges, where-by the school registers for a scholarly account, which can have self-set administrators (teachers), moderators (teaching assistants) and self-allocated accounts (students). All of the progress each student makes is ranked up into the global statistics for that school. Obviously this is a huge step into a different direction, but as an alternative to charging their users, this seems like a fair idea. I guess the teachers would also be able to set targets and homework assignments that must be completed, thus making the app an integral part to their class.