This paper reports on a number of challenges faced in tracing contributors to research projects that were originally conducted many decades previously. The need to trace contributors in this way arises in projects which focus on involving research participants in previous studies who have not been maintained on a database, or with whom the research team has lost contact since the initial study. The paper focuses, in particular, on the use of social networking sites to trace contributors and reflects on the issues raised when using these tools for this purpose. It reports on a study which attempted to trace, using the social networking sites "Facebook" and "Friends Reunited," contributors to surveys of childhood play conducted by Iona and Peter Opie in the 1950s and 1960s. Success in using the sites for this purpose was limited and it is suggested that in cohorts which include generations and communities that might use new technologies less frequently than others, additional methods to track down participants may be required.