The service does collect users' mobile phone numbers and accesses contact lists in order to locate the mobile phone numbers of other WhatsApp users.
Facebook has not disclosed its plans for the service and did not respond to a request for comment Monday.
The FTC letter makes it clear that the commission will scrutinize changes to WhatsApp.
"It sent a strong signal that the FTC is watching--and isn't fooled by the claims of Facebook and WhatsApp founders that their motives are pure," Chester said.
The letter warns Facebook that any changes to WhatsApp's data collection policies could violate an agreement it has with the FTC. In 2011, Facebook settled charges that it deceived users on "numerous occasions" about how it shared their personal information.
Under the settlement, Facebook is barred from making deceptive claims about privacy and is required to get user approval before changing the way it shares their data. The company must also undergo periodic assessments of its privacy practices by independent auditors over the next 20 years.
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