Under Article 7 of the DMA, the designated gatekeeper Meta must make its messaging services, WhatsApp and Messenger, interoperable with those of third-party providers that request it, and must publish a reference offer with the technical details and general terms and conditions of this interoperability. After presenting the details of this obligation and the context that led to its inclusion in the final stage of the legislative process, we will examine how the enforcement has worked in practice, how the market players have reacted, and the potential obstacles that may explain its limited impact on market dynamics so far. Finally, we will also briefly discuss proposals to extend the horizontal interoperability obligation to social networks.