Nearly all users got in during "free for life" periods. Whatsapp was never making anywhere near $1/user/yr. To give you an idea of just how few users were paying, in the first half of 2014, they had 450 million users and generated only $15 million of revenue [1]. Even if you annualized that to $30 million, that means about 6.7% of users were ever paying a fee to use it.

[1] http://time.com/3545911/facebook-whatsapp-earnings/