A social media brand is the sum total of customer experiences with a company, its products, services, and employees, and the way those experiences shape our perception of the company.
A CMS (Content Management System) has more of a hub and spoke model. One person puts the information (usually in the form of a document) into the repository. Others can go get it. I call it a 1:portal:1 model. Can it be collaborative? With some pains, yes. It isn’t quite real time, but can approach it. One person owns the document, even though many may have some input. The focus, of course, is on the document and the user usually cannot customize what they want to see.
An organization typically requires a CMS when it creates and publishes large volumes of content, or the publishing process is too time consuming and inefficient, or if there are so many publishers that the existing system has no standardized approach to efficiently publish, store and organize content for user consumption. Many problems including "information overload" and search engine ineffectiveness explain why content management continues to be one of the most pressing and important issues facing Internet and intranet site managers.