For global messaging sensation WhatsApp, the privacy brouhaha that followed its sale to Facebook came as a rude surprise. Soon after the $19 billion deal was announced, consumer privacy groups asked federal regulators to investigate the merger for potential consumer harms and possibly block the deal. Some users are threatening to leave the service.

WhatsApp founders on Monday tried to deflate concerns that user data may be used for advertising. But it will be hard for the messaging service to convince users who thought they had signed up to service that would never use data for targeted advertising, privacy advocates say. Any deal with Facebook comes with the baggage of the social networking giant's troubled history on privacy.

"They took Facebook's money, and now one of them has a seat on their board," said Jeff Chester, head of Center for Digital Democracy, a privacy group that along with the Electronic Privacy Information Center recently filed a complaint against the merger to the FTC.

Jan Koum, who co-founded WhatsApp with Brian Acton, will join Facebook's board once the deal closes.

Facebook has repeatedly changed privacy policies on users, having the effect of a slow boil that constantly pushes the comforts of users who are at this point too reliant on the network to leave, some consumer groups say.

The merger of Facebook and WhatsApp brings together two companies with diametrically opposing business models and philosophies on consumer data. Facebook's success is tied directly to how much data it collects about its users and sells for advertising.

WhatsApp has taken great pride in collecting as little information about users as possible. In a blog post Monday, Koum reiterated that no phone numbers or other data of its 455 million users will be shared with Facebook. The free (or nearly free) messaging app doesn't ask your name or e-mail address. There is no real sign up procedure like most subscription services and the firm will be run independently from Facebook. The other things WhatsApp won't collect:  home address, GPS location, your likes, search history. And in a followup email, Koum said messaging history isn't stored in its servers. "None of that data has ever been collected and stored by WhatsApp, and we really have no plans to change that," Koum said.

"I want to make sure you understand how deeply I value the principle of private communication. For me, this is very personal. I was born in Ukraine, and grew up in the USSR during the 1980s. One of my strongest memories from that time is a phrase I’d frequently hear when my mother was talking on the phone: “This is not a phone conversation; I’ll tell you in person.” The fact that we couldn’t speak freely without the fear that our communications would be monitored by KGB is in part why we moved to the United States when I was a teenager."

It's personal for Facebook and WhatsApp users too. And it's natural for users -- who have seen their profile pictures suddenly become public incorporated into sponsored ads--  to predict changes will be in store for WhatsApp, privacy groups say. In 2012, Facebook settled FTC charges that it deceived consumers by allegedly sharing information about users without the consumer's knowledge.

"WhatsApp should enter into a 20 year consent decree with the FTC that it will never accept advertising or incorporate Facebook marketing products," Chester said.