Koum launched the messaging app with Brian Acton in 2004 before selling it to Facebook for a colossal $19 billion in 2014.
Koum is the only founder of a company acquired by Facebook to serve on its board.
"It's been nearly a decade since Brian and I started WhatsApp, and it's been an unbelievable journey with some of the best people", Koum said in his Facebook post. "It is time for me to move on", he writes.
He continued: "I'm taking some time off to do things I enjoy outside of technology, such as collecting rare air-cooled Porsches, working on my cars and playing ultimate frisbee".
Koum's departure comes just a day before Facebook's annual f8 developer conference, where the company is looking to move beyond the Cambridge Analytica scandal and the associated privacy backlash, but this announcement might just make those efforts a bit harder - especially since it brings back memories of Whatsapp co-founder Brian Acton and his departure from the company.
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Koum's defection could put CEO Mark Zuckerberg in an uncomfortable position Tuesday, when he takes the stage at a company conference.
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 US, fake news, data leaks and more. However, the Post suggested in its report that attempts by Facebook "to use its personal data and weaken its encryption" had caused tensions between the two companies, with the recent data scandal involving Facebook and Cambridge Analytica contributing to "a climate of broader frustration with Facebook among WhatsApp employees", sources with knowledge of the matter told the news outlet.
Facebook bought WhatsApp in 2014, and back then, Koum wrote that the deal wouldn't have happened if WhatsApp "had to compromise on the core principles that will always define our company, our vision and our product".
Privacy was WhatsApp's hallmark, and especially important to Mr. Koum, who grew up in Soviet-era Ukraine.
To attract more advertisers to WhatsApp, there were also discussions about weakening the platform's end-to-end encryption, which essentially prevents anyone, including Facebook or WhatsApp, from reading the contents of messages sent over the service.
Despite the Post's reporting, Koum portrayed his departure in positive terms.
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