In the past decade, social media platforms such as Facebook and Twitter have gone from being a novelty to becoming an essential part of many people's personal and professional lives. Like previous changes in communications technology, social media poses a legal challenge. Can existing laws be applied or adapted to this new context, or does it pose new problems requiring new solutions? This article examines one aspect of this question through an analysis of the private international law issue of what law applies (or should be applied) to cross-border defamation claims on social media. Cross-border defamation raises a range of issues, including private international law questions regarding which courts should adjudicate claims and which substantive law should be applied. While the jurisdictional issues are important and have a significant impact on the issues of applicable law, there are distinct questions and concerns raised by the choice of law question for cross-border defamation on social media. Indeed, it is a topic which perhaps raises some of the most difficult issues in private international law, as well as having important broader consequences for media law and free speech regulation. At a general level, it concerns choice of law in defamation, which has proven a particularly challenging subject in practice and in proposed law reforms – at present it remains excluded from both UK and EU statutory rules concerning choice of law in tort. More specifically, it concerns defamation online, a context which might be grounds for suggesting that a further specialised rule is required – a view taken by the ECJ in relation to jurisdiction over online defamation. And finally, it concerns defamation online on social media, which raises challenging issues in terms of adapting the law to new media contexts, as well as identifying the relevant ‘public’ within which a reputation is established. These are not just difficult practical questions, arising with increasing frequency in litigation, but also problems of principle which have broader implications. As social media become increasingly important modes of socialisation and communication, greater attention will need to be paid to the question of whose law governs standards of free speech on social media platforms – an important part of the question of whose law rules ‘Facebookistan’.