Metadata is the important part here. Metadata can show who you send a message to and when. You might remember the term from the Snowden leaks, because the CIA was collecting metadata on phone calls. While WhatsApp doesn’t keep your messaging beyond the course of it trying to deliver that message (if the recipient is offline it’ll stay on WhatsApp’s servers until the message goes through), it does collect a lot of other information about you. Based on their Privacy Policy, this includes usage and log information, device information, contact information, cookies, status updates (like when you were last online), and your location if you choose to share it. They can also put that metadata together using other people’s information. For example, if you’re not sharing your contact list, but a friend of yours is and you’re in it, then they can put those two pieces of information together. It’s also worth remembering that Facebook owns WhatsApp, which means it shares data for ad targeting. You can opt out of this, but it’s a noteworthy features because the relationship between the two is going to make some people uncomfortable. None of this is bad by any stretch of the word, but it’s still worth noting.