Social networking sites (SNSs) have been suggested to facilitate risky sexual activities. However, it is unknown and of concern how SNSs such as Facebook shape risky sexual activities in developing settings such as Swaziland, the country hardest hit by HIV and AIDS. We conducted an online cross-sectional study in 2012 to explore the prevalence of multiple sexual partnerships (MSPs) and their correlates among Facebook users in Swaziland. The response rate was 44.1% (N = 882); relatively, an equal proportion of men 82.7% (341/414) and 82.9% (388/468) women had ever had sex. Of those sexually active, 44.9% of men and 30.7% of women reported having sex with someone they met on Facebook. Approximately half of the participants (61.6% men, 41.0% women and 50.6% total) reported MSPs over the past 12 months. Multiple logistic regression analysis revealed that time spent on Facebook, “finding it easier to initiate a romantic conversation on Facebook” and having had sex with someone met on Facebook were significantly associated with having MSPs (adjusted odds ratio = 1.6–3.8). The potential impact of risky sexual behaviour among Facebook users should be appropriately addressed particularly in high HIV-prevalent settings like Swaziland.