We've already learned quite a bit from Mark Zuckerberg's grilling by a joint session of the Senate Commerce and Judiciary Committees. But Zuckerberg has now revealed another fact that is sure to worry anyone who doesn't use Facebook. As many already suspected, Facebook collects data about non-users, too.

As Bloomberg reports, Representative Ben Lujan asked Zuckerberg whether or not Facebook collected data on users who did not have an account. Zuckerberg admitted they do, stating "In general we collect data on people who are not signed up for Facebook for security purposes." He doesn't recognize the term "shadow profiles," though.

Lujan rightly went on to point out that, "You've said everyone controls their data, but you're collecting data on people that are not even Facebook users who have never signed a consent, a privacy agreement." Zuckerberg fell back on the security reasoning, stating "We need to know when somebody is trying to repeatedly access our services."

It gets worse than just an admission of data collection without consent, though. Lujan had clearly done his homework and investigated how to gain access to a record of data collected about non-users. It resulted in Facebook asking non-users to sign-up for an account and therefore no data is available.

Zuckerberg is admitting something many suspected, and in doing so causes Facebook a serious problem. It is now on record that Facebook collects data about non-users and Zuckerberg has no idea how much data is stored without consent or specifically how it is used. It seems clear Facebook is going to have to unlock that data, because as Lujan stated "We've got to fix that."

Such data being made public would reveal the full extent of Facebook's tracking without consent. Zuckerberg believes the non-user data is stored simply for security purposes. However, Bloomberg points out that former Faecbook employee Antonio Garcia Martinez believes his explanation of the data use is incomplete. If it is, and it's being used for other commercial, privacy-breaking things, expect to see Zuckerberg answering many more questions most likely surrounded by a small army of lawyers.

This article originally appeared on PCMag.com.