Social networks attract increasing attention from practice and academia, yet social media networks as part of a start-up's strategic communication efforts remain under-studied. To fill this gap, we conceptualise start-up social media communication within the framework of communication management theory. We set up five hypotheses regarding the influence of different communication activities on start-ups' communication success and the moderating role of firm age. We collected data through structured questionnaires using a sample size of 244 start-ups in Germany and ran ordinary least squares (OLS) regression analyses. Results indicate that social media communication management is a significant driver of communication success. Two main factors, environmental scanning and networking, are revealed. Furthermore, we find different communication and networking practices depending on the start-up's age: In early phases, ad hoc monitoring, personal networks and business-to-consumer networking are key elements. In contrast, a long-term communication plan, professional networks and business-to-business networking are crucial in later stage start-ups.