There is a growing need to comprehensively understand how messages and online conversations construct and convey meanings especially when they are likely to be distributed across multiple social media platforms. This paper introduces a framework to address this need, the Social Semiotic Multimodal (SSMM) framework based on multimodal extensions to Systemic Functional Linguistics, a semiotic theory of language. The framework uses a set of expansion resources, to reveal how the meanings of social media messages are chained together to form online conversations. These meanings are frequently distributed across more than one social media platform. This semantic approach is exemplified using a case study, the Fairtrade Fortnight 2012 campaign. The framework is used to identify meanings associated with various social media conversations called Themed Clusters. The findings demonstrate the utility of semantic approaches for IS research and practice to analyse meanings within social media messages and how they form online conversations.