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?
WhatsApp is more problematic to leave though. Where I live, it’s the default messaging app everyone uses. I haven’t texted anyone in ages. And I can also see why it’s the default, WhatsApp is just better than the competition. It’s end-to-end encrypted, so Meta cannot read your messages directly, it has good markdown-like formatting support, it’s has a lot of features, and it’s relatively stable.
I’ve been using Telegram and Signal with friends, but honestly Telegram doesn’t exactly feel safer to me, especially with e2e encryption not enabled by default (last time I checked). And Signal is better, but sometimes just a pain to use. No Markdown-like syntax (though formatting is finally possible via GUI), it constantly keeps desyncing devices that I use once every few weeks, and we’ve had plenty of bugs with not seeing messages of eachother.
Now, I can accept that to a degree, in return for better privacy. But no way in hell are laypeople like my family going to switch. WhatsApp is too good and safe enough to remain dominant.