I use this link which will open in WhatsApp by default to send a WhatsApp message to someone who is not in my Contacts and don't want to add to my Contacts just to send a WhatsApp message.
It will be a pain in the arse to use for all your contacts and probably not worth the effort as if you use WhatsApp regularly (for an Indian there's no way around it) you will have sent a message to almost everyone on your contact list at least once and Facebook will know about it rendering the whole thing pointless.
So then you can at least minimize the number of contacts you expose to WhatsApp to the number of people you're actually in touch with regularly over WhatsApp.
People forget how many of that 400 million are illiterate and waiting to be taken for a ride.
"We can reach everyone therefore we should" mindless characters crawling like termites all over the place.