It's common sense to expect that anything you write on the internet in private may well become public one day. Just one odd click, a hack, one of your conversation members tagging in a third party.
Years ago I was a little emotionally bent out of shape. After my public moaning became untenable I began to write a private (available only to me) blog. I could conceivably have written the most horrible things about the person I felt had wronged me, but my self-check about "what if it was public?" caused me to hold back.
And eventually I came to think that if you can't stand by your words if they became public then should you be thinking those things at all? I managed to self teach that one.
The boys from Adelaide High School have had to learn the hard way.