Facebook makes a lot of noise about privacy (using their own definition, naturally, being 'preventing anyone outside Facebook from seeing your data') so it's reasonable for a nontechnical user to not expect that an acquaintance installing a "what's your pirate name arrr" app should lead to their 'share with friends only' info being acquired and sold by a data broker.
The difference is that email forwarding is part of the RFCs and IETF standards that define email, e.g.:
https://www.ietf.org/rfc/rfc5322.txt
Whereas with Facebook, while one or more parties might be bound by some agreement with the company, that doesn't create any clear agreement between users.
If you have a problem with usage implying consent to a TOS then you can make that argument, but this one is flimsy.
They see a "forward" button in their email client. Just like Facebook Messenger has a "forward" button on messages.
No one is expecting a different level of privacy in Facebook Messenger over email because of the IETF.
Edited to add: Granting for instance access to your contacts fro an app is controversial from a legal perspective in Europe. Data protection agencies at least in Germany have the opinion that you need opt-in consent from everybody in the address book to do that.
No. I would impose a ban on selling our conversations for money or for services.
Edit: to clarify, I believe users on Facebook were bartering with data that wasn't theirs for access to Facebook's services.
How does this world view reconcile with the fact that many different states have 2 party consent? Or the fact that considerable money and effort have been spent on concepts like OTR?
People expect others to have some discretion in what private information they pass on to others. Most applications have this assumptions built in mind.
You can easily forward a single email to someone in gmail, but there is not 1-click solution to send all of your emails to someone else.
That’s actually wrong. Right now, today, you can write apps for the Gmail platform that could have this ability as soon as someone authorizes it [1]. So everyone is down on Facebook, who disabled this ability 4 years ago. What about Google?
This isn't your nana forwarding chain mail.
So perhaps it's not an expectation of privacy, but more of an expectation that your private messages won't be distributed by third parties for profit.
The world doesn't just give you privacy. If you want any, you have to work at least a little bit for it.
We're getting a little ridiculous here. There are valid complaints against Facebook in all of this - this just doesn't happen to be one of them.