Many people switched their communication methods in China; due to its internet censorship. These allowed methods are easily monitored by the authorities. China blocks or slows down the secure communication channels so that people cannot use them.
WhatsApp is now blocked in China. WhatsApp is owned by Facebook and it provides encrypted communication between the two. Facebook’s social media services have been blocked in China since 2009; WhatsApp was the last product from Facebook still to be available in China.
Source: The New York Times