Introduction: This study investigated a type of post-adoptive system use behaviour that is derived from the original purpose of the system. We defined this type of post-adoptive use as a secondary system use as it is different from the primary use of the system for which the system was intended. We focused on the way that Facebook can be used to document personal lives, in addition to using it as a social networking tool, as one example of secondary system use. The study tested the factors that influence a secondary system use. Method: An online survey was distributed through the Amazon Mechanical Turk to collect responses from Facebook users. Analysis: We created a research model, expanded from the technology acceptance model, to examine how users perceive usefulness of a secondary use and then use Facebook for a secondary purpose. The model was tested with partial least squares and path analysis. Results: The research model provides a baseline for understanding the formation of a secondary system use within information systems. The findings show that users develop a secondary system use in the post-adoptive stage of system exploitation. This behaviour is largely based on their primary use experience and the perception of its usefulness for both primary and secondary use. Conclusions: This study emphasises and explains the importance of secondary system use in the development of information technology. By learning more about users' post-adoptive use behaviour, system developers and information professionals can understand changes in system use and identify directions for future development of systems.