Political advertising on Facebook is the latest in a long line of developments in campaign practice, and is a tool that has been mobilised extensively in elections around the world. In this article, we explore what we know about Facebook advertising at elections and ask what existing data from the UK Electoral Commission can reveal about current usage. Highlighting the principles behind Facebook advertising, we argue that existing metrics offer little insight into current campaign trends—posing analytical, methodological and normative challenges for academics and electoral regulators alike. Moreover, we argue that these challenges strike at the heart of debates about democratic responsibility and the degree to which governments should cede responsibility to commercial actors who may have differing understandings of fundamental democratic norms.