Social media adoption within organisations enables wider employee participation in corporate communication and rhetoric. We study the impact of social media on rhetorical practices inside organisations, namely how social media reshapes senior management communication. We study the online communication environments of eight organisations and identify two contrasting approaches in dealing with social media adoption: the closed and open model. In the closed model, organisations maintain central control and their communication platforms remain mainly one-way. In the open model, organisations develop and foster two-way interaction. The study finds that in the "open model", governance and culture of the organisation changes in order to address the shift in control and tension between top-down and bottom-up communication. Our key contribution is in rethinking rhetorical practices in the context of modern open and fluid online communication environments in organisations - rhetorical diffusion - and characterising the changes in governance and culture that enable this transition - internal ambidexterity.1