Social Networking Sites

When visiting and using Facebook, Instagram, Twitter and Tumblr, I find different variations of the same concept. All deal with getting the message across to people following you, or people you are friends with, depending on which social media platform you are using.
Facebook is, for most obvious reasons, the largest of the four. It gives users the ability to personalize your home page, including the ability to add a profile picture, post other pictures and statuses, create a public description of yourself, share a link or article, and so forth. As a matter of fact, Facebook allows the user to become free to do basically anything online. It is safe to say that Facebook is like a mix of all the other social media sites out there. The only big difference I see among the other listed social media sites is the friend feature. Whereas all the other sites listed have a one way relationship by a simple follow button, Facebook requires the two way relationship (I believe it makes sense to say that if you are X's friend, then X is also your friend).
Instagram on the other side is mainly used to post pictures so that your followers, or if you are public,  all the users are allowed to see and comment on your posted picture. Instagram is basically a blog of pictures. So why don't people stick to Facebook to post pictures and do exactly what Instagram does? Honestly, I do not have an answer, and I would love to see comments below why. My guess would be to say marketing was a big part of why.
Twitter is a very simple social media site that allows people to 'tweet' things to the followers. A tweet is defined by any, so called status, that doesn't reach over 140 characters. I personally do not have an active Twitter account (personal one, not one that gives me points in a class), but many celebrities do. In fact, even in live events such as a basketball game, or the Grammy's, the commentators of the TV program show live tweets of celebrities to show what they are thinking at the moment. The most recent time I have seen this was the NBA slam dunk contest, where all the big stars were tweeting the Zach Levine and Aaron Gordon showdown.
Tumblr was, in recent times, a very rapidly growing social media site that had features of Instagram and Twitter. Although Facebook has the same feature, people were allowed to post pictures and blogpost statuses to their followers. These picture are rarely their own pictures, but rather re-shared photos from other users. From personal experience, I never really liked the idea of a Tumblr. It was hard to use, even though Facebook was also hard in the beginning as well.