That doesn't scale nearly to the $20 billion they were bought at. Yelp, which has a similar business model to what you quoted, and is more essential to businesses than WhatsApp, only has a market cap of $3.6 billion.
I'm not sure you understand how popular Whatsapp is outside the US. No one in my country has heard of Yelp (201 million population). Everyone uses Whatsapp daily. Whatsapp has 1.5 billion active users.
Yeah, but if we're going by a business features monetization model, what would matter is the number of businesses that are signed up, not the number of users.