It doubled down on its pledge by adding encryption in 2016.
Jan Koum's exit from Facebook Inc. could prove costly.
In a comment responding to Koum's message, Facebook boss Mark Zuckerberg wrote: "Jan: I will miss working so closely with you".
Acton also recently invested $50 million in WhatsApp rival Signal, which was not lost on analysts. He has joined a chorus of former executives critical of Facebook.
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.
"I'm leaving at a time when people are using WhatsApp in more ways than I could have imagined", Koum wrote. The app has about 1.5 billion users worldwide.
Mr. Koum's departure followed an internal debate over whether to put advertising in WhatsApp, a move that Mr. Koum opposed, as did Mr. Acton, according to a person familiar with the matter. By 2014, the tiny company had nearly 500 million users.
Acton left Facebook past year to start a nonprofit.
But even in the early days, there were signs of a mismatch. It now has more than 1.5 billion users.
A major part of WhatsApp's popularity is its encrypted messages which are stored on users' phones, not on company servers, making it more private.
Koum announced his departure on Monday; he said it was time for him to move on, and made no mention of any tension with Facebook. WhatsApp has never made any profits. "WhatsApp will remain autonomous and operate independently".
Eighteen months later, the promise not to share data evaporated. But in coming months, Facebook has discussed plans to introduce ads to its "Status" feature, which allows users to post images and videos that disappear within 24 hours, much like on Instagram or Snap Inc.'s Snapchat, the person said.
WhatsApp recently agreed with the United Kingdom government to stop handing data over to Facebook on investigation of its data policies. Dwoskin reports that, during one change to WhatsApp's terms of service, Facebook bosses wanted to create user-specific profiles that could reach across platforms like Facebook and Instagram-and thus be used "for ad-targeting or for Facebook's data-mining".
Another issue centered on advertising.
Who will lead the messaging service in the interm isn't clear. WhatsApp is also now piloting its UPI-based mobile payment service in India.
Another point of disagreement was over WhatsApp's encryption.
Despite WhatsApp's eye-popping acquisition price of $19 billion, the messaging service reportedly hasn't prioritized generating revenue for its corporate parent.