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?
I don’t use Facebook but I have a handful of friends on Instagram that like to send me funny stuff. They all don’t really know each other so I find myself passing on memes like some kind of 1930s phone operator connecting calls.