I don't remember paying even the $1 to them ever and I've been using WhatsApp for a good few years. Every time the expiration date approaches, it automatically gets pushed away. Either they were just giving away subscriptions intentionally or I'm just one lucky bastard.

So, No, they never had 1 billion dollar paying customers.

They used to claim to charge you after 1 year, but I never got charged due to a combination of changing phone numbers, them not asking for it, etc.