First of all, what you guys do is amazing and I am very grateful and lucky to be a part of this educational expansion. I teach language and service skills in China and am a huge fan of Quizlet. It's SOOOOO easy to make quizzes, integrate audio, and generate flashcards for students on their site.
They obviously are lacking what you guys have: progress tracking, reports, and student management. If the two of you combined - WOW - powerhouse. Online courses could be created so easily by teachers, and data could be so easily managed and understood by parents and administrators.
Are you working on collaboration with Quizlet? Integrating some of their features? Right now It's quite clunky on Schoology to make quizzes. There's no bulk import function from simple OFFICE or WORD files where most teachers do their planning and creation.