At present whatsapp has estimated 15m per annum in revenue.
It has earned this through a yearly service charge: it is free for one year and is then charged at 1 per year thereafter.
But its growth is so fast it is adding one million new users a day that it could reach a billion users within two years. Even if a quarter of those users paid 1, that would yield 250m per year for a firm with just 55 employees and few overheads.