A really useful piece of work done by @simplymeasured on how social media teams are typically structured and where they fit into the rest of the marketing team.
In my experience, the role of Content Manager is key. That person's role is to curate, not always to create content. They act as a librarian who captures all of the content that an organisation is creating now, re-format it to use in many social channels and then request content where it doesn't exist.
The best content is often written by subject matter experts, rather than comms teams. So, the role of the content manager is requester, analyser, editor, chaser...They know what content works for which audiences and in what formats. They should be driving the strategy as well as using their social media skills to amplify the content that the business is already creating, by sharing it with influencers and using all of the social channels.
Their blog has this report and lots of other useful stats. The state of social media teams.