Tons of people use IG and I think they pretty much know that it's them, the other guy and whatever number of contractors monitoring chats. They just don't care.

Maybe one of the most helpful parallels is with mail. I think US and other countries have strict laws about mail communication privacy. Someone can in theory open your mail but it's strictly regulated and not done in a total way.

Also I do think talking about future malicious government prosecuting people based on what was collected previously is actually a good one. But just talking about privacy may be a little too vague.