They're trying to monetize WhatsApp through their Business API. They have a directory of Business Service Providers that can provide API access and integration services. Their partnership model is very convoluted and flawed, though, and it's hurting adoption.
Not yet, they are obviously moving into merging WhatsApp with Messenger/Instagram Direct, but it looks like these recent actions against FB (US Congress questioning monopolistic practices, FB having promises it would not merge WhatsApp with FB Messenger at the time of acquisition, and various privacy investigations from EU countries) are making FB go very very slow on this.
Therefore, so far FB only collects contacts and improves its social network (in the technical sense) with WhatsApp.
https://developers.facebook.com/docs/whatsapp/