We present the first comprehensive evidence of how app usage affects academic performance and early career outcomes. App usage is contagious: a one standard deviation (around 3.5 hours per day) increase in roommates’ app usage raises an individual’s own app usage by 5.8%, with substantial heterogeneity across students. A one standard deviation increase in app usage reduces GPAs by 36.2% of a within-cohort-major standard deviation and lowers wages by 2.3%. The effect of roommates’ app usage is over half the size of an individual’s own usage effect. High-frequency GPS data reveal that high app usage crowds out time in study halls and increases late arrivals at and absences from lectures.