Very weird you don't know this: Even if you haven't heard of somebody who has paid for Whatsapp service (you must be surrounded of linux/opensource people), it already has sold millions of dollars. MILLIONS. *MILLIONS* (read it again).
It was the top selling application in several countries with revenues even bigger than Angry Birds! YES: PEOPLE PAID FOR IT. There are 2 billion of Whatsapp messages PER DAY, Whatsapp Inc never reveals the number of users
In conclusion, they don't need to sell your number to anyone. They are already millionaires.