What is Postmodernism?

Postmodernism is pretty much what we live in today. It is what were going through right now. Postmodernism defines why things are the way they are. Well, that's at least what I understood. Why we live in the world we do now. All of this technology, social media stuff. I think when writing about Postmodernism we tend to show events from the past that give reasoning to why things are the way they are now. There is always an explanation for everything right?
We are pretty much surrounded by all of this pop culture and social media craze. I wonder how this all came about? What started this? I wonder, who thought it was a great idea to think of something like Instagram. What in past events lead to something so great but so mind consuming? What is its explanation? This social media app has most teens hooked.