You do if you’ve been acquired for $19 billion. The founders couldn’t have been naive enough to think that Facebook and its shareholders didn’t have a need to monetize such an enormous investment. Facebook is not a charity, it’s a business.
Facebook monetizes Whatsapp by limiting its growth. You can easily think of all the “social” functionality Whatsapp could have added in competition with FB and Messenger.
That is what I am seeing now here in India, folks in my circle barely post anything to facebook now, all pictures, status messages are on WhatsApp now.