Social networks provide new spaces in which to explore one’s identity and how it is presented to others. To do so, it is essential to study how they affect the construction of one’s self-concept and perceived affection in adolescence. The principal objective of this study was to analyse the congruence or incongruence of self-concept within (online) and outside (offline) the social networks and their relation to affect. The participants were 350 adolescents (41% males), between 14 and 19 years of age. They completed the AF-5 to evaluate self-concept and the PANANS to evaluate affect states or personal emotions. Both questionnaires were completed twice, taking into account online and offline situations. We found differences in all the dimensions of self-concept in 24% of the participants, and only in the social dimension in 51.4% of the participants. The participants who showed differences between their online and offline self-concept obtained higher scores in wellbeing in comparison to those who maintained similar self-concept. Social media allow adolescents to experiment with a different self-concept which influences their affect.