The article investigates how the theory of horizontal effectiveness of fundamental rights has been applied in judicial decisions on content moderation on social networks in Brazil and Germany. It is noted that, in Brazilian law, there is no clarity on whether the application of the doctrine of direct effectiveness in matters of content moderation would be in line with the jurisprudence of the Federal Supreme Court. In German law, on the other hand, recent decisions by the Regional Higher Courts and the Bundesgerichtshof signal a tendency to declare the ineffectiveness of private contractual clauses that restrict fundamental rights to freedom of expression. In both jurisdictions, the interpretation of the terms and conditions of use of platforms poses methodological challenges for the theory of horizontal effectiveness.