I’d guess the simple answer may be that people don’t care/know, but for those that have ditched Facebook for whatever reason, yet still use Instagram, it’s a little surprising.
Is it partly a result of it being a more focused/limited form of social media that filters out some of the stuff people didn’t like from Facebook?
Yep. And by everyone I literally mean everyone. Every iPhone has whatsapp installed, my workplace uses it, every single one of my friends use it, my parents use it and my both grandmothers use it. Hell my pet gerbils probably have whatsapp too.