This is precisely while I'm on instagram: There's no pretense of anything being private. It's like the best parts of Facebook (cool pictures/info from people who interest me) without the bullshit (TDS-fueled ramblings).
I'll be out of there as soon as a federated alternative pops up and gets the least bit of traction.
But let's keep this serious: Instagram isn't a tool for secure messaging. It is a tool to publish images, mostly public images.
A person might very well decide to move sensitive communication off Instagram and continue to post their cat videos on Instagram.
A valid reason for not using Instagram however is to lead by example and weaken the network effect of Facebook.
Facebook is already in panic because users are leaving the platform so IMO now is a good time to test out alternative solutions :-)