Some Facebook users have been giving the social network access to their phone activity in exchange for money as part of a research project since 2016, according to a Tuesday report by TechCrunch.

Facebook, which critics say isn't doing enough to protect the privacy of its users, has been giving people between the ages of 13 and 35 a payment of $20 per month plus referral fees for their phone and web activity. Facebook is able to access this data after users install a "Facebook Research" VPN app.

The company is able to view web searches, location information, private messages in social media apps, and other data, Guardian Mobile Firewall security specialist Will Strafach told TechCrunch. The study's participants are even asked to screenshot a page showing what they ordered from Amazon, according to TechCrunch. 

Beta testing services BetaBound, uTest and Applause helped distribute the app and don't initially mention on the sign-up pages for the social media study that they're letting Facebook access participants' data. But if minors try to join the study through a page administered by Applause, they have to get their parents to sign a form that mentions Facebook's involvement in the study. 

The "Facebook Research" app is similar to Facebook's Onavo Protect app that Apple banned from the App Store for violating its privacy rules, according to TechCrunch. Facebook removed the Onavo app in August. 

Facebook didn't immediately respond to a request for comment, but it confirmed to TechCrunch that it was running a research program to learn about people's phone usage.

"Like many companies, we invite people to participate in research that helps us identify things we can be doing better," a Facebook spokesperson told TechCrunch. "Since this research is aimed at helping Facebook understand how people use their mobile devices, we've provided extensive information about the type of data we collect and how they can participate. We don't share this information with others and people can stop participating at any time."

BetaBound, uTest and Applause didn't immediately respond to a request for comment.

Cambridge Analytica: Everything you need to know about Facebook's data mining scandal.

Security:  Stay up-to-date on the latest in breaches, hacks, fixes and all those cybersecurity issues that keep you up at night.