In particular, the FTC said, regulators will closely scrutinize corporate claims that Americans' data has been or will be "anonymized," in light of substantial research showing that it can be trivial to reverse-engineer a person's identity from anonymized datasets. The FTC said it has also sued companies in the past for collecting more data than consumers have consented to providing or that retain user data for indefinite periods of time.

Last year, the FTC settled with Flo Health, the maker of a fertility app used by more than 100 million consumers, for allegedly sharing pregnancy and other data with Facebook and Google despite saying the information would only be used to facilitate the app's normal function. (In a statement at the time, Flo said the settlement was "not an admission of any wrongdoing.")

The FTC's warning is a reminder that digital information ostensibly collected for one purpose may wind up being used for others through the country's sprawling and loosely regulated data economy, in which personal data is constantly bought and sold, or chopped up and repackaged with other information.

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According to a 2014 FTC report, some data brokers may hold thousands of pieces of information on every consumer in the United States. FTC Chair Lina Khan has said the agency is considering drafting new regulations to rein in what she has called a "commercial surveillance" industry that profits from intrusive data collection and sharing, while Congress is currently considering a bill that could enshrine a baseline data privacy right for all Americans at a national level for the first time in US history.