After a year of asking Facebook to explain its approach to privacy, at least two US senators still have questions.

Sen. Mark Warner, a Democrat from Virginia, and Sen. Edward Markey, a Democrat from Massachusetts, each took the company to task Wednesday in light of reports that it paid teens as young as 13 to download an app that could track their every action on their phones.

In separate press releases, the senators pointed to growing frustration with the social media giant for not being clear about its data collection practices. For his part, Warner said he is drafting a bill to require major companies to get informed consent from users whose data becomes the target of market research.

"I have concerns that users were not appropriately informed about the extent of Facebook's data-gathering and the commercial purposes of this data collection," Warner wrote in a letter to Facebook CEO Mark Zuckerberg.

Facebook didn't immediately respond to a request for comment. But a spokesperson told CNET earlier Wednesday that Facebook didn't share the data it collected and users knew what they were signing up for. Less than 5 percent of the users were teens and all signed parental consent forms, the spokesperson said. 

Warner also asked Facebook for a fuller accounting of how it came to approve the apps, which had deep access to track activity on mobile phones, including apps that Facebook was competing with.

Markey took issue with the ethics of targeting teens with an offer they might not have the maturity to access.

"It is inherently manipulative to offer teens money in exchange for their personal information when younger users don't have a clear understanding how much data they're handing over and how sensitive it is," he said.

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