Encrypt all the things!

Facebook "unintentionally" uploaded the email contacts of more than 1.5 million users without asking permission to do so, the social network has admitted.
The data harvesting happened via a system used to verify the identity of new members,
Facebook asked new users to supply the password for their email account, and took a copy of their contacts.

https://www.bbc.co.uk/news/technology-47974574