That doesn't mean that you'll start seeing ads on WhatsApp. The messaging service is sticking to its promise of never offering ads on its platform. The data will instead be leveraged to show you targeted ads on Facebook, offer better friend suggestions, and fight spam. And if you don't want the service to share your data with Facebook, you can opt out.
In a blog post announcing the changes, WhatsApp reiterated its commitment to user privacy, stating that all messages will stay private. WhatsApp won't be able to read them, nor will Facebook, or any third-party advertisers:
We won't post or share your WhatsApp number with others, including on Facebook, and we still won't sell, share, or give your phone number to advertisers.
By coordinating more with Facebook, we'll be able to do things like track basic metrics about how often people use our services and better fight spam on WhatsApp. And by connecting your phone number with Facebook's systems, Facebook can offer better friend suggestions and show you more relevant ads if you have an account with them. For example, you might see an ad from a company you already work with, rather than one from someone you've never heard of.
Even though ads aren't coming to WhatsApp, the service has mentioned that it is looking at ways to engage its userbase with businesses:
In the future, we will explore ways for you and businesses to communicate with each other using WhatsApp, such as through order, transaction, and appointment information, delivery and shipping notifications, product and service updates, and marketing. For example, you may receive flight status information for upcoming travel, a receipt for something you purchased, or a notification when a delivery will be made. Messages you may receive containing marketing could include an offer for something that might interest you.
We do not want you to have a spammy experience; as with all of your messages, you can manage these communications, and we will honor the choices you make.
The Facebook family of companies will still receive and use this information for other purposes such as improving infrastructure and delivery systems, understanding how our services or theirs are used, securing systems, and fighting spam, abuse, or infringement activities.