Koum said in a post on his Facebook page that he is taking time off to pursue interests such as collecting air-cooled Porsches, working on cars and playing ultimate Frisbee. "I'll still be cheering WhatsApp on - just from the outside".
Koum reportedly takes issue with the company's attempts to "weaken [WhatsApp's] encryption" and "use its personal data".
Stanford alumnus Acton and Ukrainian immigrant Koum co-founded WhatsApp in 2009.
But with Facebook keen to score some returns on its investment, the social networking giant eventually got WhatsApp to alter its terms of service to give it access to users' phone numbers, as well as other data that it could use to benefit its broader business.
Koum's departure follows other reports of high-level internal disagreement at Facebook. "Thanks to everyone who has made this journey possible". "Those values will always be at the heart of WhatsApp", Zuckerberg wrote in a comment on Koum's Facebook post.
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But unlike WhatsApp, which will only apply the new law to its European users, Facebook plans on rolling out changes for its users worldwide.
Koum's boss, Facebook CEO Mark Zuckerberg, in a comment on Koum's post, wished him well and said he'll miss working closely with Koum. Facebook acquired his popular messaging app for $16 billion in 2014. The harvested data was later used by Cambridge Analytica used data to target voters in the U.S. general election in 2016 raises tough questions for both companies.
The Washington Post, which earlier reported details of Koum's departure, said Koum would leave both WhatsApp and Facebook's board. Acton isn't involved with WhatsApp anymore, having left it in November to later join Signal Foundation.
This development came amid growing concerns about Facebook's handling of personal information since the social network's admission in March that the data of millions of users was wrongly harvested by political consultancy Cambridge Analytica.
Koum cited his Ukranian upbringing during communist rule as a reason why "the desire to protect people's private communication" was one of WhatsApp's core beliefs, in a 2016 post announcing the app's end-to-end encryption. The departure also comes after he unloaded billions of dollars worth of Facebook stock. The app has about 1.5 billion users worldwide.