In a statement, a Facebook spokesperson explained why contacts are uploaded: “The most important part of apps and services that help you make connections is to make it easy to find the people you want to connect with. So, the first time you sign in on your phone to a messaging or social app, it’s a widely used practice to begin by uploading your phone contacts.

“Contact uploading is optional. People are expressly asked if they want to give permission to upload their contacts from their phone – it’s explained right there in the apps when you get started. People can delete previously uploaded information at any time and can find all the information available to them in their account and activity log from our Download Your Information tool.”

One user, Dylan McKay, reported that for the period October 2016 to July 2017 his logs contained “the metadata of every cellular call I’ve ever made, including time and duration” and “metadata about every text message I’ve ever received or sent”.

Many other users reported unease at the data they discovered being logged, including the contacts of their address books, their calendars, and their friends’ birthdays.

https://www.theguardian.com/technology/2018/mar/25/facebook-logs-texts-and-calls-users-find-as-they-delete-accounts-cambridge-analytica