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Privacy groups ask US FTC to stall Facebook's $19bn WhatsApp deal. Reuters

Privacy advocates have asked US regulators to stall Facebook's $19bn acquisition of the hugely popular messaging service WhatsApp until there is clarification that its 450 million users' personal data will remain protected from advertising and marketing.

WhatsApp, a service that allows mobile phone users to send each other messages, has committed to not collect user data for advertising purposes. It stores users' mobile phone numbers, but does not collect user names, emails, and other contact information.

In addition, WhatsApp does not show ads on its service and charges some of its users a $1 annual fee to use the service. But Facebook, the leading social network with 1.2 billion users, generates the majority of its revenue through ads that target users by gender, age and other traits.

There is no guarantee that the WhatsApp commitment would be honoured once the service becomes part of Facebook, non-profit groups, Electronic Privacy Information Center and the Center for Digital Democracy, said in their filing with the Federal Trade Commission (FTC).

Despite assurances by WhatsApp and Facebook that the privacy policies would not altered, the groups said that Facebook has in the past modified an acquired company's privacy policies, such as that of Instagram, the photo-sharing service that Facebook bought in 2012.

Regulators must stipulate that Facebook "insulate" WhatsApp user data from access by Facebook's data collection practices, according to FTC filing.

Pursued by Reuters, the FTC refused to comment.

The complaint asks regulators to investigate the deal "specifically with regard to the ability of Facebook to access WhatsApp's store of user mobile phone numbers and metadata."

"WhatsApp users could not reasonably have anticipated that by selecting a pro-privacy messaging service, they would subject their data to Facebook's data collection practices," reads the filing.

"As we have said repeatedly, Whatsapp will operate as a separate company and will honour its commitments to privacy and security," Facebook said in its response to the filing.

Facebook shocked the technology industry last month when it announced its decision to buyout five-year old WhatsApp for $19bn (£11.35bn) in stock, cash and RSUs.

The purchase of WhatsApp dwarves Facebook's other social messaging service purchase - Instagram - which cost the social network less than a $1bn.