The aim of this study is to examine the role of Instagram usage duration and addiction on the happiness of students at the University of Gonabad. In terms of approach, this study is quantitative, and in terms of method, it is a survey-based, descriptive–correlational study. Data were collected using the Oxford Happiness Questionnaire and the 15-item Persian version of the Instagram Addiction Scale. The statistical population comprised all students enrolled at the University of Gonabad during the 2024–2025 academic year, from which a sample of 150 students was selected using convenience sampling. To analyze the data, a threshold regression model was employed, and the Davies test was conducted to examine the significance of the slope change. The results indicated that the duration of Instagram use exhibits a threshold effect on students’ happiness. Specifically, daily usage up to approximately 1.84 hours was associated with increased happiness, whereas usage beyond this threshold led to a significant decrease in happiness levels. Moreover, Instagram addiction showed a significant negative relationship with happiness, while marital status had a positive effect, and gender did not significantly influence happiness. These findings suggest that controlled and moderate use of social media platforms may contribute positively to mental well-being. In contrast, excessive and addictive use can have detrimental psychological effects. The study highlights the importance of monitoring social media consumption and promoting awareness programs to encourage balanced usage among university students. Furthermore, the application of threshold regression provides a nuanced understanding of how different levels of Instagram usage differentially affect happiness, emphasizing the critical point beyond which the negative effects of social media become prominent. The results have practical implications for educational institutions, counselors, and policymakers in developing interventions to enhance student well-being and prevent the adverse outcomes associated with excessive social media engagement.