In an attempt to track whatsapp metrics, Facebook wants to connect your whatsapp phone number with Facebook for better ads experience.
Company itself said, "updated documents reflect that we’ve joined Facebook". And it is. The company said, "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".
The company also noted their future plans, the company may test some features to provide notification about fraudulent transaction, or getting notified by an airline about a delayed flight, in the next several months.
If you are an existing user of whatsapp, you can choose not to share your whatsapp account information with Facebook.