The New York Times details how Facebook has shared user data without their consent to other companies. At least:
- Microsoft's Bing search engine could see the names of your friends
- Netflix, Spotify and the Royal Bank of Canada(!) could read your private messages
- Sony, Microsoft and Amazon could get your email addresses
- Pandora and Rotten Tomatoes got to hold on to all the personal data Facebook leaked earlier
- Yandex had access to your unique user IDs
- Apple got access to your contact numbers and calendar entires, even if you disabled all sharing
- Other phone manufacturers -- at least Blackberry and Huawei -- got access to your friends and probably other personal information
According to the investigation, there was practically no oversight on how that data was used. An early Facebook investor, Roger McNamee sums it nicely in the article:
“No one should trust Facebook until they change their business model.”
What should you do? If you're in Europe, ask Facebook to delete all your data you think it shouldn't have. Thanks to the European GDPR regulation, they will honor that request. (If you're not in Europe, lobby for better legistlation.)
Stop using all the Facebook products you can. Even if you can't delete your account, consider using alternatives when possible.
Specifically, use WhatsApp instead of Facebook Messenger, if you have to continue using Facebook's messenger services. Facebook can read your Messenger messages (and shares them with others), but can't read WhatsApp messages.