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?
By that token, that’s the other side of the question, what does Instagram do for those still using it? I’ve seen a few replies that give some interesting insights on this, but Instagram’s never struck me as being as heavily integrated as WhatsApp given its focus, so I’d guess maybe a mix of memes and, admiration, to put it nicely?