Koum's post doesn't include an explanation of exactly why he's leaving other than "doing things I enjoy outside of technology".
He said he would still be "cheering WhatsApp on - just from the outside". At the time of the acquisition, Koum and Acton said Facebook had assured them that WhatsApp could remain an independent service and would not share its data with Facebook.
Facebook COO Sheryl Sandberg in comments replied, "The work you've done building WhatsApp has connected so many people around the world".
Facebook did not address the reasons for Koum's departure from the company. "Thanks to everyone who has made this journey possible", he added. "Those values will always be at the heart of WhatsApp". Compared to the billions that Facebook was making with targeted advertising, WhatsApp recorded a net loss of $232.5 million in the first half of 2014.
To recall, Facebook is now facing its toughest time with user data privacy after Cambridge Analytica.
WhatsApp was founded by Jan Koum and Brian Acton, with the latter departing the company back in 2017 several years after Facebook had acquired the company.
He did not provide a departure date and did not confirm whether he would be joining Mr Acton who left WhatsApp in September to start a foundation.
Since the Cambridge Analytica data harvesting scandal hit in March, there have been growing concerns about Facebook's handling of personal information. And Acton and Koum may be followed by an even larger falloff, as the WaPo report suggests November as a mass-exodus point, when original WhatsApp staffers will be allowed to exercise stock options.
The Washington Post, which earlier reported details of Koum's departure, said Koum would leave both WhatsApp and Facebook's board.
Acton, who left WhatsApp in November previous year, has donated US$50 million to Signal, a messaging app made by Open Whisper Systems, and is now executive chairman of the non-profit Signal Foundation.
Facebook CEO Mark Zuckerberg posted on Koum's timeline, writing he was thankful for what Koum taught him about "about encryption and its ability to take power from centralized systems and put it back in people's hands".
Koum also plans to step down from Facebook's board.