The application of crowdsourced social media data in flood mapping and other disaster management initiatives is a burgeoning field of research, but not one that is without challenges. In identifying these challenges and in making appropriate recommendations for future direction, it is vital that we learn from the past by taking a constructively critical appraisal of highly-praised projects in this field, which through real-world implementations have pioneered the use of crowdsourced geospatial data in modern disaster management. These real-world applications represent natural experiments, each with myriads of lessons that cannot be easily gained from computer-confined simulations. This paper reports on lessons learnt from a 3-year implementation of a highly-praised project- the PetaJakarta.org project. The lessons presented derive from the key success factors and the challenges associated with the PetaJakarta.org project. To contribute in addressing some of the identified challenges, desirable characteristics of future social media-based disaster mapping systems are discussed. It is envisaged that the lessons and insights shared in this study will prove invaluable within the broader context of designing socio-technical systems for crowdsourcing and harnessing disaster-related information.