The world's biggest social network is in hot water over privacy rights yet again, with another controversy involving user data revealed this week.

Facebook revealing they've been using an app, called "Facebook Research," to track the behavior of its users, many teenagers. The app was a voluntary download, with users 13-to-35 receiving compensation in exchange for allowing Facebook - and possibly 3rd party app developers - to track nearly all their smartphone activity. This includes what apps were being used, what websites were visited, who they were texting, and what those messages said.

Most users got $20 monthly gift cards from Facebook in exchange for using the app, and some were also given paid referral fees. But when the news hit, it prompted an immediate backlash on social media and elsewhere, with critics accusing the company of taking advantage of minors who may not have been aware of what they were agreeing to. Many users also compared this incident to another recent revelation involving Facebook tracking smartphone behavior on users. And they were also called out for using a Virtual Private Network - or VPN program - to track their users online.

Facebook says it didn't do anything wrong. The company notes in a statement that all users voluntarily signed up for the Research app, and were free to opt out at any time. They also note that all the users were compensated, writing in a statement, "it wasn't spying as all of the people who signed up to participate went through a clear on-boarding process asking for their permission and were paid to participate."

But other tech giants disagree. Apple says the level of access Facebook gained by using the app is a clear violation of Apple's developer guidelines, and confirms it was taken off the Apple store within hours of the story breaking. It's still available for Android users, but not in the Google Play store, for a similar reason: Google says the access gained by the app violates the company's privacy policy.

Lawmakers in Europe recently passed the world's toughest online privacy laws, fining big companies billions of Euros for violating users' rights. And now there are rumors revelations like the "Facebook Research" app could have lawmakers on Capitol HIll following suit.