title: How do the social media monitoring sites fetch the huge number of user posts?
body: <p>There are many social media monitoring sites in the market. I am very curious about how do the sites fetch the posts of such a huge number of users. How do they know which user's posts should be fetched?</p>
  
  <p>For example, if one site needs me to log in with my Facebook account, and it just fetches/analyzes my or my friend's posts. That would be reasonable. But I tried several social media monitoring services several days ago, I found that there are massive amount of data fetched, users of all kinds are included.</p>
  
  <p>How do the services know which user's data should they fetch? If they fetch all the posts of a certain social site, how do they achieve that? Doesn't the social site's API always prohibit apps from fetching data with large amount?</p>