Haven't they already monetized on WhatsApp? They've already been able to gather a massive amount of social graph data and personal details, phone number, location information.

If they made any amount of value on WhatsApp data, the users are never informed. I think the only way to really know is to join as an employee and find out what they do/did with it. The only thing we are aware of is advertising when we see it, everything else is nebulous.