We study the effects of advertising disclosure regulations in social media markets. Theory generates ambiguous predictions about the effects of regulations on the equilibrium amount of advertising content, user engagement and welfare. Using data from a large sample of Instagram influencers in Germany and Spain and a difference-in-differences approach, we empirically evaluate the effects of German disclosure regulations on post content and follower engagement. We measure whether posts include suggested disclosure terms and use text-based approaches (keywords, machine learning) to assess whether a post  is sponsored. We show a substantial adoption of disclosure after regulations, but also an increase in sponsored content including undisclosed sponsored content. We also find reductions in engagement, suggesting that followers were likely negatively   affected.