Despite growing evidence of the effects of social media on the mental health of adolescents, there is still a dearth of empirical research into how adolescents themselves perceive social media, especially as knowledge resource, or how they draw upon the wider social and media discourses to express a viewpoint. Accordingly, this article contributes to this scarce literature. Six focus groups took place over 3 months with 54 adolescents aged 11-18 years, recruited from schools in Leicester and London (UK). Thematic analysis suggested that adolescents perceived social media as a threat to mental wellbeing and three themes were identified: (1) it was believed to cause mood and anxiety disorders for some adolescents, (2) it was viewed as a platform for cyberbullying and (3) the use of social media itself was often framed as a kind of 'addiction'. Future research should focus on targeting and utilising social media for promoting mental wellbeing among adolescents and educating youth to manage the possible deleterious effects.