This is probably spot on, and the bit I missed when reading. So they share your phone number and contacts (or Facebook and Whatsapp use the same hash off your p

This is probably spot on, and the bit I missed when reading. So they share your phone number and contacts (or Facebook and Whatsapp use the same hash off your phone number and share that), but Facebook promises not to put the number you have in Whatsapp on the website.