Twilio did $365 million in quarterly revenue. WhatsApp made up 7% of sales. That means WhatsApp accounted for over $25 million, or a $100 million run-rate. Facebook spends $100 million just for account verification on WhatsApp!
Twilio gives WhatsApp and big customers massive discounts on per message pricing. Helps keep their volume on the platform which gives good negotiating position for their upstream pricing.