Facebook does indeed do that. One of the big reason of paying 22B for whatsapp is so that they could retrieve the entire friend graph of WhatsApp and use that to drive Facebook + Instagram MAUs.
The bigger the network, the more value a user gets, and the deeper the lock in.
It has been shown over and over again that both Facebook and Google will go to extreme lengths to know about their users’ lives and target them with precision ads. They are advertising companies foremost.
There are no mutual friends or any other link it could make apart from linking my contact list with their number.