Facebook, Instagram- Distinctions?

I use Facebook daily. I understand how it works, how it’s used, etc.

I have never used Instagram.

As I understand it, Facebook purchased Instagram.

What I don’t understand is what the point of the services being different actually is. Why would someone post Picture A on Facebook, Picture B on Instagram, and Picture C on both?

All the other main social media sites I get, but the Instagram thing escapes me.

All of your questions can be answered by one simple sentence: my mom’s on Facebook, she’s not on Instagram.