Some magnet schools have a competitive entrance process, requiring an entrance examination, interview, or audition. Other magnet schools select all students who apply or use a lottery system, or a system combining some elements of competitive entrance and a lottery.
As of February 2016, WhatsApp had 1 billion monthly active users, up from April 2015 when the company announced over 800 million MAU. The company's growth in India is also pretty impressive at around 5 million additional MAU per month (60M in Aug '14).
The app does require phone functionality, so it won't work on all devices. WhatsApp is free to use for the first year, with a cost of $0.99 per year. The basic ease of use, wealth of features and ballooning user base of more than 450 million people explains why Facebook was willing to pay $19 billion to acquire it.