Very nicely explained by Susan. But I really think Facebook is still the king in most of the cases. Twitter builds more of your personal profile. Instragram is an awesome retail promotions tool and a direct selling place. Plus, it’s very powerful in reaching people if you use the following hashtags no matter what your post is about. Without these hashtags, I could get only 50 likes on an interesting image. But with addition of the following likes, I could easily get hundreds of likes and the post becomes available globally. Relevant or not, it’s all in the reach of your post at the end of the day. More reach more followers. More likes.