WhatsApp founder and Chief Executive Officer Jan Koum is leaving Facebook Inc. (FB.O, just a few years after his messaging app was acquired by the social-media giant for US$22 billion.

Koum posted his plan to depart on his Facebook page, saying the decision was emotional. His mobile-messaging tool, which has been kept separate from Facebook’s main social network, its Messenger app and other businesses, is known for offering end-to-end encryption, meaning only the sender and recipient can see the content of messages. The app has about 1.5 billion users worldwide. The news received a quick response from Facebook CEO Mark Zuckerberg.

“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,” Zuckerberg wrote. “Those values will always be at the heart of WhatsApp.”

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. The other co-founder of WhatsApp, Brian Acton, also left recently -- and last month posted the #DeleteFacebook hashtag during the social network’s scandal over user privacy.

When asked who will replace Koum at the head of WhatsApp, or whether he plans to remain on Facebook’s board, the company said it had no comment. The company also declined to comment on whether Facebook plans to start advertising on WhatsApp — a business model the founders promised they would never use.

--With assistance from Nathan Crooks.