BackgroundDuring the COVID-19 pandemic, adolescents' use of social networking sites/apps has surged, and their mental health and quality of life have also been significantly affected by the pandemic and its associated social-protection measures. The present study first examined the prevalence of social networking sites/apps use and social networking addiction, the mental health status, and the health-related quality of life among Hong Kong adolescent students. We further investigated the associations of the youths' daily use of social networking sites/apps and their social networking addiction with their mental health and quality of life during the pandemic.MethodsA total of 1,147 students (age = 15.20 ± 0.53 years) recruited from 12 randomly selected local secondary schools in Hong Kong participated in a questionnaire survey in classroom settings between January and June, 2020, right after the COVID-19 outbreak. The questionnaire includes demographic characteristics and scales that measure social networking sites/apps use and social networking addiction, mental health, and quality of life.ResultsApproximately 46.4% of the participants reported using social networking sites/apps often or very often, and 7.8% met the criteria for social networking addiction using Bergen's Social Media Addiction Scale. The prevalence of mild to extremely severe depression, anxiety, and stress among the adolescents stood at 39.6, 37.5, 48.8%, respectively, and the participants' physical, social, and school functioning were lower than the norms of healthy adolescents before the pandemic. Participants who used social networking sites/apps but for <3 h per day (excluding students who never used social networking sites/apps) showed significantly fewer problems of depression, anxiety, and stress than did those who spent more than 3 h per day on social networking sites/apps. Social networking addiction was found to be consistently associated with poor mental health and health-related quality of life.ConclusionThis study provides important evidence supporting the potential protective effect of guiding adolescents to use social networking sites/apps appropriately in order to mitigate their negative emotions during contexts such as that of the pandemic; it further points to the need to provide extra support to promote the well-being of young people, especially those in disadvantaged situations (e.g., non-intact family) during and after the pandemic.