Which marketing strategy did Duolingo pioneer that revolutionized language learning app engagement?

In the world of language learning apps and linguistic tools, marketing strategies often leverage unique psychological principles to attract and retain users. This poll tests your knowledge about one of the most successful marketing approaches used in the language learning industry that revolutionized how these products are positioned to consumers.

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The 'streak' gamification system that creates FOMO (fear of missing daily practice)
The 'translation marketplace' where users earn money by translating content
The 'linguistic immersion' subscription model with native speaker conversations
The 'vocabulary first' approach that prioritizes common words before grammar