How Duolingo uses streaks and notifications to keep users coming back to the app; 5M of Duolingo's 30M+ DAUs have one year streaks and 70% have weeklong streaks (Ben Cohen/Wall Street Journal)

https://www.wsj.com/tech/personal-tech/duolingo-streaks-notifications-app-spanish-bc87d6e4?st=y9q82nw5dbsruas&reflink=desktopwebshare_permalink
http://www.techmeme.com/240615/p6#a240615p6