http://www.washingtonpost.com/blogs/the-switch/wp/2017/07/31/sheryl-sandberg-whatsapp-encryption-actually-helps-governments-combat-terrorism/

Sheryl Sandberg defended Facebook's use of encryption in its popular messaging service WhatsApp, telling a BBC radio show that what limited data remains accessible can be useful to law enforcement as it seeks to thwart terrorist activity.

When communications are encrypted, only the sender and intended recipient can read the message. But information about an encrypted conversation, like who is contacting whom, would still be available to governments during a terrorism investigation, even if the contents of the conversation would not.