There are many, many people that have joined social platforms because they saw other people doing it and decided that they should to. Each one is different and has a different purpose. I have some background in marketing, so I am intimately familiar with most of the large platforms. To better understand them, imagine each like a social setting in real life.
-Facebook is like a neighborhood barbecue or a pub. It's casual setting. Most of the people there are going to want to talk about themselves and what's going on in their lives. If anything related to politics or football has happened recently, expect to hear about it.
-Twitter is like the highschool-hallway at lunchtime - if you listen, you can hear a conversation about almost anything. Ultimately, Twitter is a news source. It started falling away from being a traditional "social" platform a while back and became much more of an real-time information tool when the company failed to narrow down it's content algorithm. It's kind of like trying to drink out of a fire-hose, you just get too much at once.
-Instagram is like a student art gallery. Some pieces are incredible to look at, others are not. Instagram was made to share life through pictures. Unlike in real photography, Instagram is designed so that anyone can quickly filter and easily edit photos to create beautiful art.
-Snapchat is a platform for sharing photos and video in 1-10 second increments that are able to be seen for no more than 24 hours. It creates a sort of "sharing the moment in the moment for a moment" feeling.
-LinkedIn is the conversation you have around the office water-cooler. Note, you present yourself differently here than you would in a more casual setting (like on Facebook or Twitter). You talk about news, articles and books you've read, friends you have that just got promoted or moved to a new company, and find mutual acquaintances. LinkedIn is an excellent tool to boost personal credibility as well as develop professional skills through their online courses.
There is also Pinterest (the world's largest image-based search engine), Tumblr (a micro-blogging site), Musical.ly (designed to create and share music videos), YouTube (you know what YouTube is), and plenty of other smaller sites. What platforms you use highly depends on what your purpose for them is.