Existing users will be prompted in the next few days to agree to the new terms. If you don't want to share your data with Facebook, however, you can opt out. Opting out is simple: when you are notified to agree to the new terms, tap the read more pane at the bottom. You will then see a checkbox; unticking it will disable sharing data with Facebook. The checkbox reads:
“Share my WhatsApp account information with Facebook to improve my Facebook ads and products experiences. Your chats and phone number will not be shared onto Facebook regardless of this setting.”
By now, some of you will be wondering how you opt out if you've hastily pressed agree already. Luckily it's still possible... for 30 days. If you already agreed but want to opt out, head over to Settings > Account > Share my account info. If you don't want to share the data with Facebook just go ahead and untick the box.
It should be noted this is only a partial opt out, and the Facebook family of companies will still receive and use the information for other purposes such as improving infrastructure and delivery systems, understanding how the services are used, securing systems, and fighting spam, abuse, or infringement activities.