Earlier quoted context omitted.
> Freely sharing your friends' data without their permission may be a GDPR violation. Absurd, how would an email provider or social network even work if you couldn't enter your friends contact information.
There is an exception for data that is required for the functioning of the service. You need your friends email address to use email, but do you really need their birthday or a sentiment analysis of their opinion of cheesecakes?
If you go back to the Wild West of the Facebook apps, shortly after platform launch there was an app for everything - apps for fancy birthday cards with birthday reminders, as well as polling apps telling you which one of your friends is the biggest cheesecake lover.
Every piece of data can be spun into being essential.