Duolingo was designed around the psychology of a streak.
Luis von Ahn, who had sold reCAPTCHA to Google, opened Duolingo to the public in 2012. What kept people coming back was not the lessons but the streak, a count of days practiced in a row that drops to zero the day you miss one. By the end of 2025 about 15 million daily users had a streak of a year or longer.
Duolingo on Brandseum