This article considers the dynamic relationship between language policies, practices and ideologies in a multilingual Facebook group in Luxembourg. The group under focus, "Free Your Stuff Luxembourg", was created to facilitate the cost-free exchange of consumer goods between members located in Luxembourg. The article traces the development of a language policy for a group that facilitates communication between people of diverse nationalities in an officially trilingual country, where French, German and Luxembourgish operate as administrative languages and English plays an increasingly important role. Part one analyses the development of the group's official language policy by group administrators, showing progression from an implicitly English language policy to an explicitly multilingual policy, incorporating a strong place for Luxembourgish. Part two considers how the language practices of group members relate to this official language policy, using a quantitative analysis of the language(s) of group posts across three periods from February 2011 to April 2012. This analysis shows a shift from predominantly English language practices to a balance between English and Luxembourgish, and finally a dominance of Luxembourgish. Part three investigates a further influence on language policy development, the language ideologies of group administrators and members, as expressed in language ideological debates within the group. The results provide several insights in relation to language policies in the new media, addressing the role of new agents of language policy (group administrators) in regulating language use in this context, the processes by which individuals police each other's language use online, and the extent to which language practices in online environments can be managed, if at all, through language policy activity.