1) The document discusses social machines, which are computational systems involving both human and machine participants working together. Examples mentioned include Wikipedia and citizen science projects. 2) Key aspects of social machines are discussed, including their collaborative and open nature, use of stories and narratives, and focus on empowering human participants. 3) Sustainability of social machines over time is an important challenge, as maintaining volunteer participation can be difficult, as seen with the declining contributor numbers on Wikipedia. Designing social machines to be reactive and allow for improvisation may help with long-term sustainability.