Q:

How is social media data unstructured data?

I recently began reading up on big data, and how there are tools like hadoop or BigInsights that can manage both structured and unstructured data. 
Social Media Analytics is something that can be done on BigInsights, and it takes unstructured data and analyzes/structures it accordingly. 
This got me wondering, how is Social Media Data unstructured? For example, the information you can receive on tweets can be called using the Twitter REST API, and returned to you in a structured JSON format.
So isn't Social Media data already structured? If so why do you need a platform that manages mainly unstructured data? 

A:

Some make the distinction „semi-structured”, too.
But the point is the ability to query the data. Yes, Tweets etc. usually have some structure. But it's not helpful for analysis.
Given an ugly SQL schema, you could indeed run a query like
 SELECT AVG(TweetID) FROM Twitter;

but that functionality is useless in practise. And that is probably why the data is best considered unstructured: you do not benefit from squeezing it into a relational schema.
Beware of buzzword bingo with big data, though. More often than not „supports unstructured data” actually means „does not benefit from structure in your data (by using indexes) but rereads data every time”

