The concept of 'authenticity' is highly valued on social media sites (SMSes), despite its ambiguous nature and definition. One interpretation of 'authenticity' by media scholars is a human's congruence with online portrayals of themselves (e.g. posting spontaneous photographs from their lives, or using real biodata online). For marketers and 'influencers', these patterns of behaviour can achieve certain gains: sales for a business, or success of a campaign. For existentialist philosophers, using 'authenticity' as a means to an end is against its very definition. In this paper, I investigate what SMS users are looking for by their supposed 'authentic' portrayal online. My experimental approach draws upon empirical data from the Instagram social media site. Using machine learning techniques, descriptions and features of posts - including subjects, captions, and contexts - will be categorised and aggregated. I will then interpret these findings, drawing upon work by Taylor, Golomb, and Guignon, whose works on authenticity are based on mid-20th century existentalists. I argue that the existentialist ideals on authenticity are not necessarily present in contemporary SMS use. I will also argue that the popular interpretation of authenticity on SMSes can be self-defeating, when it seeks to turn the 'for-itself' into an 'in-itself'.