SaaS (Software as a Service) solution just like Buffer

Suppose you were given a task to create a SaaS (Software as a Service) solution just like Buffer, which is used to schedule social messages. It posts social messages to Facebook, Twitter, LinkedIn and Google Plus. Each user account will be connected to 10-20 social profiles. Imagine a client like BBC who generates thousands of contents each hour and those contents needs to be shared in social media. The message can be scheduled on a certain period or can be shared instantly. The proposed solution will have to handle clients like BBC or every news channels available in the world where everyone is sharing anything in seconds. You've to handle 10 thousands of social shares each minute in different channels. How would you design such thing?

Please design and architect a system and you don't have to code it now, but design the overall system so that any person can understand. What technologies you would use and everything in between. Please be brief as much as possible.