Social networks use is related to the occurrence of eating disorders (ED). In this study, we experimentally evaluated the effect of exposure to social networks and stereotypical images of the thinness ideal on ED symptomatology and analyzed the mediator role of anxiety in this process. A sample of 321 young adults of both sexes (166 females) were randomly assigned to two experimental conditions: high and low exposure to the thin ideal. Our results indicate a decrease in self-esteem in the group exposed to the thin images and an increase in anxiety. We found the effect of the images on self-esteem is completely mediated by the increase in anxiety. No changes in body satisfaction or drive for thinness were found. Our study shows how brief exposure to images and profiles representative of the thin ideal seems to influence participants’ self-esteem. These results show the need to raise awareness of the possible consequences of social media, as well as to promote a healthy use of social networks.