With interaction across social networking sites (SNSs) becoming more prominent, the ability to accurately judge another's personality through these digital platforms is an important area of investigation. A number of studies demonstrate that SNSs can be an effective means of communicating information on personality (Evans, Gosling & Carroll, 2008). Much less research has examined the online cues that people use when forming their impressions of another's personality. The current research adopted a mixed methods approach to examine 1) accuracy of first impressions formed on the basis of viewing a person's Facebook behaviour and 2) how judgements are formed. Findings suggested that the traits of openness and conscientiousness can be judged most accurately through observing Facebook behaviour. To explore the cues used in forming personality judgements, thematic analysis was undertaken. This revealed six information ‘cues’ including; (1) vocabulary of target, (2) photographs, (3) written online interactions, (4) relationships with others, (5) health status and, (6) occupational status. Findings are discussed in terms of Funder's Realistic Accuracy Model (1995, 1999) along with limitations and suggestions for future research.