Well, Digital Marketing or Online Media industry is quite complex when it comes to traditional organizational structures.
It’s been almost more than a decade since the inception of online world or evolution of social media so expecting a proper structure like a traditional company it quite irrelevant and we are coming to an era where your job titles don't actually define your core work profile which can differ from what you are designated.
However coming you question which especially talks about jobs in social media, I wouldn't confuse you by talking about structures but simply gives you different titles which are not confirmed or specific and might or may differ from companies to an agency with their strength, size, and culture.
Social Media Strategist – based on company or department goals, defines a high-level plan for social media engagement
Social Media Manager – executes social media for a product, topic or channel.
Social Intelligence Professional – studies and reports on what is happening with your social channels, content and audience
Social Media Events Manager – the person who social media at physical events and organizes online social media events
Social Media Policy Manager – typically large companies, defines and enforces social media policy
Media Manager – deals with platforms that handle video and audio files and streaming
Blogger – writes blog posts, might be a percentage of another job such as product marketing manager
Brand Ambassador – often a regular employee, like an engineer, with specific expertise (ex: a product) trained on social media who helps in social media channels like forums and Twitter, typically volunteer beyond normal job duties
Community Manager – responsible for the health of a community branded or public where health means good discussions and questions getting answered
Community Moderator – makes sure users are following the rules (ex: no fowl language) and helps users with questions about the functionality of the forum, often out-tasked because this should be a 24 x 7 job
'All the best'