Social media can amplify your voice but how come it doesn't happen very often?

I know that there are thousands of examples, but considering that there are billions of Facebook posts and tweets every day, the number of the stories actually being amplified is insignificant.

Which made me think: how does this happen? Is it random? Do you need to have man followers, which will share your posts to their followers and so on? Should you post sensitive or controversial topics, which are more likely to make others share? Does it help to be aggressive and share the same post repeatedly on several social media platforms?