I think most people do care about privacy, but they're not being given much of a choice here. Their friends are on Facebook, so the only way to share pictures with all of them is through Facebook.
From my point of view, I do care about privacy, beside I don't use mobile phone, I don't want to. Since whatsapp isn't an option for me, messenger app is only way to be in contact with friends, family. (I have iPod Touch) Considering I'm traveling often, I need to use it. I'm not fan of it, but I'm not against either. In the end we are product of Facebook and #deleteFacebook is a joke.
You can use Signal on an iPod touch.
They care, but what they’ve shared is likely not what they actually care about keeping private.