The aim of this paper is to shed light on the dark side of social media and its negative effects on social cohesion. The study used descriptive method to achieve its aims by classifying and explaining the negative effects of social media into three categories: on societies, on families & individuals - especially children, teenagers & youth that affect directly or indirectly on social cohesion. The findings showed that social media threatens directly the structural features of the elements of social cohesion such as culture, identity, language, citizenship and belonging. On the other hand, it affects social cohesion indirectly through the appearance of phubbing, syntopia phenomena, intellectual invasion & extremism, low productivity & wasting official working hours, unemployment, increasing divorce rates, families' breakdown and marital infidelity. In addition to the appearance of  some mental and psychological diseases for teenagers and children such as FOMO, anxiety, Facebook depression, psychological loneliness, cyber bullying, jealousy, violence, deviant behaviors, low School attainment & low academic performance, besides other diseases like insomnia, diabetes, obesity & visual problems.