Facebook CEO and co-founder Mark Zuckerberg.

Facebook might know your heart rate even if you're not on the social network. 

At least 11 popular apps sent personal data to the world's largest social network, including information about when a user was having her period or what real estate listings a person viewed, according to testing from The Wall Street Journal. Using Facebook software built into these apps, developers were able to record a user's activity and then hand over this information to the social network even if the user didn't log into the app via Facebook or were a member of the social network. 

Heart-rate app Instant Heart Rate: HR Monitor reportedly sent a user's heart rate to Facebook. The tech giant also reportedly knew when a user got her period because she recorded it in the app Flo Health Inc.'s Flo Period & Ovulation Tracker. Realtor.com sent Facebook information about the real estate listings a user viewed, according to the Journal.

The revelation is the latest among a series of privacy concerns that have rocked Facebook, which could face more government regulation. It also highlights the trove of data Facebook collects from other apps that have tens of millions of users.

The software built into the apps includes an analytics tool made by Facebook that allows developers to see data about a users' activities and target those users with Facebook ads. 

A Facebook spokeswoman said in a statement that sharing information across apps is "is how mobile advertising works and is industry standard practice."

"The issue is how apps use information for online advertising. At Facebook, we require app developers to be clear with their users about the information they are sharing with us, and we prohibit app developers from sending us sensitive data," she said in a statement. "We also take steps to detect and remove data that should not be shared with us."

Some of the data shared appeared to violate Facebook's business terms, which tell developers not to send "health, financial information or other categories of sensitive information." Facebook said it uses data from apps to improve the ad experiences for its users and advertisers.

The Journal, which tested more than 70 popular smartphone apps, discovered that personal data was being sent to Facebook by using software that allowed them to track this online activity. 

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