Over 30 years ago, when I first started reading and participating in newsgroups on the web (rec.humor, rec.sport.baseball, etc.), I was cautious at first. One reason was that I saw some quoted email to the effect that,
Isn't it wonderful that we can carry on our affair now using email and our spouses won't know about it.
That message had gone viral, to the extent that "going viral" happened in those days. It was circulated as a warning to all that anything we write on the internet can, and likely is, on some server somewhere. Nothing is private. I have tried to keep that lesson in mind myself ever since then.
If you need more of a warning, watch this very brief (two minutes?) video: