Last week, a U.S. Senate subcommittee questioned Apple and Google on their handling of personal data. This week, Facebook joined Google and Apple on the hot seat.

A major initiative designed to improve consumer privacy on the Web — the proposed “Do Not Track” bill — could be hard to apply to mobile devices, regulators said. The bill would allow consumers to opt out of having their online activity tracked.

David Vladeck, director of the U.S. Federal Trade Commission’s Bureau of Consumer Protection, says the Do Not Track bill is designed to cover online movements, not geolocation, which might need its own protection.

While Do Not Track protects consumers from being tracked when they move from one website to another, Vladeck said, it’s not always clear within an app when this is happening.