Other than with Facebook's messenger, your WhatsApp social graph is in your phones addressbook. If Facebook's WhatsApp service were to be gone tomorrow you could just install the next biggest messenger and beyond minor initial fragmentation all your friends would still be there.
This wouldn't work with the Facebook Messenger, since this social graph is not stored by the user.
The Facebook website itself is no longer particularly useful to me, other than that it is very good at events (both private and for public event discovery). The groups feature is also handy.