Facilitated by technologies enabling a large number of networked individuals to collaborate voluntarily on translation tasks, translation crowdsourcing isa new translation procurement model which relies on crowds of Internet users willing to engage in translation activity at the request of a company or organisation. Taking the case of Facebook translation, with specific reference to the community of Polish Facebook user-translators, this study seeks to understandthe motivation underpinning contributions that are typically without financial reward, especially when the call for translation is made by a for-profit entity. A mixed methods research design involving netnography, online surveys and an observational study with elements of remote usability testing and contextual inquiry was incorporated to collect the data on the community of contributors, their translation activity and use of the Facebook collaborative translation platform. The analysis of the data suggests that the Polish Facebook user translators are motivated by a number of factors, both personal as well as social, which primarily contribute to the satisfaction of their needs of competence, autonomy and relatedness. The studied user-translators perceived the Facebook initiative as an opportunity to practice skills and effect change for the better while collaborating with others and experiencing fun. However, the flaws and limitations inherent in the purpose-built Facebook collaborative translation platform frequently prevented the translation activity on Facebook from being carried out as intended. This was found to undermine the satisfaction of needs and thus negatively affect the user-translators’ motivation. Based on these findings, the study characterises the motivations of user-translators in translation crowdsourcing in for-profit contexts and explains how motivation to contribute is affected by the translation platforms provided for the purpose. A set of guidelines for the design of such platforms is offered to organisers of translation crowdsourcing initiatives for consideration in future. Keywords: translation, crowdsourcing, motivation, collaborative translation platform