Whatsapp has end to end encryption. But the metadata is not encrypted, and Facebook uses that data to track you. What is in the metadata? If I'm not mistaken it's the recipient, the time and probably the IP address of where you sent it. So if you start talking to someone new in Whatsapp, it shouldn't be a surprised when Facebook/Instagram will suggest you add them as friends/follow them based on that data. It doesn't know what you send to you're ex on a Friday at 2AM, and it doesn't know that you what you sent to your friend the next day, from her home WiFi.
