Duolingo (DUOL) plunged 21% premarket Thursday after management said it would boost spending to drive long-term user growth, sparking margin concerns. Q3 revenue jumped 41% year-over-year to $271.7 million, beating estimates, but investors expressed concerns about profitability. CEO Luis von Ahn said the company is prioritizing strategic investments. Shares are down 20% year-to-date.