Despite growing concerns about the addictive potential of social networking sites (SNSs), little is known about the precise neural, cognitive, and emotional processes underpinning compulsive SNS behaviours, such as excessive checking of SNSs. Recent evidence points to the important role of reward in SNS behaviours and one avenue to examine reward processes related to SNSs is the use of behavioural paradigms that allow for the measurement of implicit motivational responses, such as the approach avoidance task (AAT). The AAT has been successfully utilised to capture changes in unconscious reward processes in substance use disorders and other behavioural addictions, with faster approach reactions to addiction-related stimuli reflecting increased wanting/urges to have/consume the reward. In the present study 411 young adults completed an online Visual Approach/Avoidance by the Self Task (VAAST) with social media and control logos as well as other subjective (explicit) measures of reward experience related to SNSs. Our results showed that across participants SNS logos elicited strong approach reactions (compared to control stimuli) and that stronger SNS approach tendencies predicted more frequent SNS checking. Importantly, increased approach motivation was not associated with more problematic use. However, both checking frequency and problematic use were related to alterations of explicit reward processing, including the subjective experience of SNS urges or wanting. We conclude that changes in automatic approach motivation towards SNS stimuli are common in most SNS users, which suggests that implicit imbuement of social media with reward has become pervasive among young adults. Problematic SNS use however may be more reliably indicated by changes in explicit reward processing, such as subjective wanting.