Platform work represents an important challenge for the ‘Danish model’ of unionisation. Using interviews and ethnographic data, this article analyses the strategies of the Danish grassroots union movement the Wolt Workers' Group, representing principally migrant couriers using the food-delivery platform Wolt. This study is an attempt to map an emergent form of flexible labour organisation based on horizontal, informal online networks while supported in different ways by established unions. We term this strategy of balanced autonomy and support ‘social media unionism'. Wolt couriers' attempts at grassroots organisation via social media is an important and understudied issue, especially their complex relationship to union actors. The ‘social media unionism' explored in this article allows for the formation and maintenance of nimble grassroots mobilisation among workers that are otherwise hard for unions to reach, such as migrants platform workers. We argue that this strategy holds both great possibilities and challenges for the labour movement.