Collects and sells data
Able to be monitored
Does not track your location
What Quizlet says, "Quizlet is the easiest way to practice and master what you’re learning. Create your own flashcards and study sets or choose from millions created by other students — it’s up to you. More than 50 million students study with Quizlet each month because it’s the leading education and flashcard app that makes studying languages, history, vocab and science simple and effective. And it's free!"
What SGS says, "Great for studying!"