"It is time for me to move on", Koum said in the post.
The departure comes almost four years after Facebook completed its acquisition of the messaging service for $22 billion, the largest purchase in its history. "I've been blessed to work with such an incredibly small team and see how a insane amount of focus can produce an app used by so many people all over the world". WhatsApp is said to have been comfortable sharing some data, but refused to create a "user profile that was united across Facebook's multiple platforms".
Amid ongoing scrutiny over Facebook's privacy and security standards, WhatsApp founder Jan Koum announced Monday that he's leaving the company.
The Washington Post reported earlier that Koum is exiting the company after clashing with Facebook over strategy, and that he also plans to leave Facebook's board.
"I will miss working so closely with you", Zuckerberg wrote.
Koum's post doesn't include an explanation of exactly why he's leaving other than "doing things I enjoy outside of technology". Koum had issues with Facebook's approach to privacy, user data, and encryption, unnamed sources "familiar with internal discussions" told the Post.
WhatsApp, a pun on the phrase "What's up?", only stores messages on users' smartphones and not on the company's servers, making the service more private and hard to hack.
Facebook's handling of user data has been at the heart of a scandal in recent months after it became widely publicized that political consultancy firm Cambridge Analytica was able to access the personal data of millions of Americans without their express permission. Acton left the company previous year and has since become an outspoken opponent to Facebook's handling of user data.
The man who Mark Zuckerberg says taught him about the power of encryption is leaving the Facebook family. 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".