Please stop this. This is a huge violation of your friend's privacy. Just because a friend has shared something with you on facebook doesn't mean they expect it to be in someone's personal cambridge analytica forever.
To be clear, if I want to create the equivalent of a data-hoarder bunker, I don't think there's anything wrong with that. I do, however, agree that sharing things with people that couldn't see it themselves (expanding the original audience) isn't something anyone should ever do. I'd also like to point out that once someone removes something from Facebook (or any other service, for that matter), the authenticity of copies of that information is now debatable - I could scrape a bunch of stuff and show people, but you have no way of proving whether or not that information is authentic. For all you know, my script makes any posts%5 super racist.
As for the less technical people, they tend to be significantly more annoyed at Facebook when I tell them that their old comment threads / messages are likely incoherent junk.
Where is the reasonable expectation of privacy?
If you share anything at all over Facebook, you should expect it to be in someone's personal dossier of you, forever. It's not always going to be just the people who know you and presumably bear you good will, either.
That's exactly why I don't post photos on Facebook.
No, they shouldn't, and this why we have privacy laws.
Please tell me how you expect to enforce such laws without intrusive surveillance on everyone's networks and storage that would entirely defeat the purpose of having them.
"Three can keep a secret, if two of them are dead."
Even if Alice sends a file to Bob that is entirely safe from Eve while in transit, Alice has no recourse whatsoever from within the channel if Bob then turns around and just hands it over to Eve once he recovers the plaintext. Facebook is an untrustworthy recipient. Once you hand over any data, you lose your absolute control over them.
They have the capability to hand over your data to untrusted third parties, there are no safeguards in place to prevent them from doing so, no surveillance in place to detect when they have done so, and no effective recourse for any individual who feels that they may have been injured by it. We have sufficient evidence to believe that Facebook has done it in the past (Cambridge Analytica being just one well-publicized example), without even needing to know whether it was intentional, accidental, or paid business. I therefore conclude that they are still doing it now, and will continue doing it in the future.
Anything you say or do or remember in front of Facebook can and will be used against you in the court of commerce. The machines will process it all to squeeze the pennies out, and your privacy is an issue only insofar as it may impede the flow of data you willingly hand over to them in the future.
The friend shared with Facebook, and Facebook intends to store the data forever. The friend shared via Facebook with the GP, and GP could go and read/view the data as often as they wished via Facebook.
Yes, GP shouldn't subsequently share this data with other parties, and GP should take care to secure this data. But why shouldn't GP store the data for their own use, though? Is this any different than archiving emails from the friend?
Now, morally, I could maybe understand where you're coming from. It's more of a "jerk move" than it is subverting a technical promise.
But I backup my Telegram messages occasionally through their data export tool. Are you proposing that I cross-reference my own backups with messages that get deleted from our chats? Same thing with WhatsApp.
Assuming I'm not commercially monetizing those backups, I consider it well within my rights to have a copy of conversations I've held with people in the past. And in fact, it may be by design that I don't want them to manipulate the "cloud copy" of our conversation in the future.
We're talking correspondence logs, not private investigators. Similarly, GDPR applies to a company tracking birthdays but not the one on your toilet. That your visitors can see all your friends' birthdays is not a personal Cambridge analytica, that comparison doesn't make any sense.
Should I delete that we talked to each other today in a few days when it is no longer relevant? As it is, this will be here in perpetuity, doesn't that seem dangerous as well? (I'm genuinely curious where you'd draw the line since your example isn't in line with what virtually anyone else would feel.)