Which social media advertising innovation by Duolingo in 2021 led to a 40% increase in user acquisition and became a case study in linguistics product marketing?

In the evolving landscape of language learning and linguistic tools, social media advertising has transformed how language products reach global audiences. This poll tests your knowledge about a groundbreaking social media advertising campaign that revolutionized how language learning apps market themselves to potential users worldwide. Can you identify the innovative approach that changed the industry?

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Their 'Passive-Aggressive Owl' TikTok campaign featuring the mascot humorously stalking users who missed lessons
The 'Accent Challenge' on Instagram where celebrities attempted to speak in different language accents
Their LinkedIn campaign offering free premium subscriptions to professionals who completed 30-day language challenges
The 'Translation Reaction' series on Twitter showing real-time reactions to machine translation fails