What is the main component of Social Media expert job description?

A social media person can belong to any of the following categories or traits or can have all of these badges on his/her profile to be called as an expert. 

Social media job role can be divided into following 5 categories:

1) Strategy and planning

2) Creative content

3) Community management

4) Performance media 

5) Analysis and reporting

not everyone can be an expert in all the above traits, and not everyone can be work on all at the same time. 

Now it totally depends on the structure within the agency on how they want their team members to work. There are places where you might be doing everything by yourself, in some agencies, you will be a part of the content team and rest will be managed by other team members and so on.