South Africa's HIV prevalence among young people remains among the highest in the world. A cross-sectional study was carried out in 2012 to estimate prevalences of sexual risk behavior and hazardous alcohol use (HAU) (via the Alcohol Use Disorder Identification Test) as well as to investigate potential associations between these outcomes and social media use. In all, 4485 students (mean age 15.66 years, SD 1.39) at 46 secondary schools in informal settlements in Cape Town and Port Elizabeth completed mobile-phone-assisted, self-administered baseline questionnaires within a cluster-randomized trial. In all, 312 females (12.5 %) and 468 males (23.5 %) screened positive for HAU (AOR = 1.98, 95 % CI 1.69-2.34). 730 males (39.9 %) and 268 females (11.8 %) reported having had two or more partners in the last year (AOR = 3.46, 95 % CI 2.87-4.16). Among females, having a Facebook account was associated with reported multiple partnerships in the last year (AOR = 1.81, 95 % CI 1.19-2.74), age-disparate sex in the last year (AOR = 1.96, 95 % CI 1.16-3.32) and HAU (AOR = 1.97, 95 % CI 1.41-2.74). Using Mxit-a popular mobile instant messaging application-was associated with higher odds of reported multiple partnerships in the last year among both males (AOR = 1.70, 95 % CI 1.35-2.14) and females (AOR = 1.45, 95 % CI 1.07-1.96) and with HAU among both males (AOR = 1.47, 95 % CI 1.14-1.90) and females (AOR = 1.50, 95 % CI 1.18-1.90). Further longitudinal and qualitative research should explore in more depth the observed links between social media and risk behavior.