it's not really a social platform, per sey, I guess. sure, a lot of people use it that way, but ultimately, it's just a way of sharing pictures. Benni has a really good collection of pictures on there that's worth checking out. Personally, I've been using it for a long time and have built up quite the variety of shots myself. Basically, you take a picture, post it, and people like them, people who really like your pictures (or just trying to whore themselves out for followers) follow you.
It's actually a pretty fun app, some cool filters for editing and it saves the pictures you post to your camera roll.
There's also a number of other web based apps that can link into your account that offer different services, such as;
Statigr.am - find out your stats for everything from most liked picture to people you like the most to your most active followers and even when you post pictures the most broken down by day/hour.
instacanv.as - buy and sell your instagram shots printed on actual framed canvas ranging from 12"x12" to 36"x36"
extragr.am - one of many 3rd party web interfaces for your instagram since they don't have one of their own
there another one (I forget what it's called) that can download all of your shots to your computer in a convenient zip file