Facebook has been preparing for this for years:
- cross-app communication between Messenger and Instagram
- Messenger API enabling business communication on Instagram
- ad management for Instagram and Facebook through a centralized platform
- WhatsApp business accounts linking to Facebook accounts
- Facebook shopping in-app in WhatsApp
These are all small and there are many more examples, but an overarching theme is to integrate FB's products as much as possible. It's not only to increase adoption and revenue, but also to make them appear inseparable.