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?
My younger sisters (22and 23) have never had a Facebook account but are into Instagram and Snapchat. To some degree an age difference is at play. I’m 30 and it’s crazy how differently we’re involved with the internet.