Koum's departure follows in the footsteps of co-founder Brian Acton leaving the company last September. The two men, both privacy advocates who started their encrypted messaging app almost a decade ago, joined Facebook after it acquired WhatsApp in 2014 for $22 billion, making them both billionaires. Acton, who already left WhatsApp has turned major critics of Facebook in recent times.
He said he was leaving to pursue interests such as working on his vehicle, playing ultimate frisbee and collecting Porsches.
Mark Zuckerberg, Facebook's chief executive, said in a comment on Mr. Koum's Facebook post that he would miss working with Mr. Koum.More news: Messi hits three as Barcelona clinch league title at Deportivo
But Koum also may have been embroiled in a rift with Facebook management over the parent company's voracious appetite for personal information and WhatsApp's dedication to user privacy, according to the Post report.
"I'm grateful for everything you've done to help connect the world, and for everything you've taught me, including about encryption and its ability to take power from centralized systems and put it back in people's hands", Mr. Zuckerberg wrote.
Koum added, " I'll still be cheering WhatsApp on - just from the outside".
Acton left WhatsApp late past year and joined in the growing backlash against Facebook, endorsing a campaign that encouraged users to delete their profiles from the social network.More news: President Trump a Nobel Peace Prize laureate? South Korea's leader thinks so
WhatsApp, a pun on the phrase "What's up?" grew in popularity in part because its messages are stored on users' smartphones and not on the company's servers, making the service more private and hard to hack.
Facebook has been accused of being lax and allowing the transfer of its users' information to companies who then target them with ads, along with revelations of Russian election manipulation in the USA, fake news, data leaks and more.
Koum's departure comes at a crucial time at Facebook as the company reels from revelations of Russian election manipulation, fake news, data leaks and more. Mr. Koum is up for reelection as a Facebook director, according to Facebook's proxy filing earlier this month.
Koum and Acton started WhatsApp in 2009, charging US$0.99 annually for private communications and gathered nearly 500 million users by 2014, according to the Washington Post. At that time of buyout by Mark Zuckerberg-led social media giant, WhatsApp also promised users that these values wouldn't be compromised.More news: Wall Street braced for plummeting sales of Apple iPhone X
Koum and Acton applied to work at Facebook, but failed.