Facebook is being hit with growing backlash over the revelation that it paid teenagers in order to monitor their online activity to gain valuable insights into its rivals.

The Facebook Research program, which began in 2016, has drawn the ire and scorn of critics and parents in the wake of Apple's decision to ban the data-monitoring app from all iOS phones. Apple said Facebook's research app was a "clear breach" of their agreement with the Cupertino, Calif. company. The pushback from Apple, which revoked Facebook's Enterprise Certificate, has escalated the war between the tech companies and wreaked havoc at Facebook because all of its internal, employee-only apps cannot be accessed.

"I find this behavior shameful. Taking advantage of people who do not understand the value of the data they generate. Targeting kids of all people! As a parent, I am appalled. I am so happy I stopped using Facebook years ago," Claudiu Musat, a research director for data, analytics and AI at Swisscom, said on Twitter.

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The Facebook Research program provided users between the age of 13 and 35 a monthly stipend so the Menlo Park, Calif. company could surveil their mobile activity, including private messages in social media apps, chats from instant messaging apps, web browsing activity, photos and videos sent to others and web searches — according to TechCrunch, which first revealed the program's existence. The backlash to the research app arrives the same week that 100 public health advocates sent the beleaguered social network a letter expressing outrage over a separate initiative that allowed underage users to rack up huge gaming bills on their parents' credit cards thanks to Messenger Kids.

Facebook COO Sheryl Sandberg, who has been under fire over the Cambridge Analytica data scandal and her role in the company's opposition research of financier George Soros, said Wednesday that all participants who opted in consented to have virtually all of their phone habits monitored by the tech giant. “The important thing is that the people involved in that research project knew they were involved and consented,” she told CNBC in a Wednesday interview.

Another user wondered how Facebook was not in more trouble over this latest privacy incident.

"Did they have a way of verifying that the parents who gave permission were actually their parents?" wondered Sara Danner Dukic in a series of tweets. "And what about teens who said they were 18 but were lying? Did they have a way of verifying that they were of age to legally consent?"

Less than five percent of the participants in the program were teenagers, according to Facebook's public statements, and they all signed parental consent forms. However, the social network has not released data on exactly how many users participated in the program.

Late Thursday, Facebook confirmed to Fox News that its enterprise certification had been restored by Apple. "We are in the process of getting our internal apps up and running. To be clear, this didn't have an impact on our consumer-facing services," the Facebook spokesperson told Fox News via email.

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"How can children authorize [Facebook] to collect data? Thirteen is clearly not old enough to consent to what was going on. This is beyond offensive. [Facebook] is a dirty, vicious company," another user, Susan LaDuke, wrote on Twitter.

Fox News reached out to Facebook for answers to these questions and will update the story as needed.

Facebook isn't the only company in hot water over its misuse of the Apple Store to gather data on users. Up until recently, Google was running an app called Screenwise Meter that rewarded users with gift cards in return for allowing the search giant to monitor their web traffic and data, according to TechCrunch.

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However, the search giant's program differs from Facebook's Research app in several key ways, according to a source familiar with the matter. Screenwise did not ask users for "root" access, which would have allowed it to monitor encrypted data. Also, only people age 18 and over could sign up their household to use it (although they could invite underage panelists in their household to partake, parental consent was required). In all, fewer than 10,000 devices downloaded the app globally.

When asked about the app, Google provided Fox News with the following statement:

"The Screenwise Meter iOS app should not have operated under Apple’s developer enterprise program — this was a mistake, and we apologize. We have disabled this app on iOS devices. This app is completely voluntary and always has been. We’ve been upfront with users about the way we use their data in this app, we have no access to encrypted data in apps and on devices, and users can opt out of the program at any time."

On Thursday evening, Apple announced that it was blocking Google from running its internal iOS applications, such as company employee-only transit and food apps, because Google was also in violation of its rules. "We are working together with Google to help them reinstate their enterprise certificates very quickly," an Apple spokesperson told BuzzFeed News.

Google had its enterprise developer certificates restored late Thursday, according to CNBC.

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Meanwhile, Facebook is being questioned about how its program could properly safeguard the privacy of users, especially children.

BBC reporter Dave Lee was able to sign up on Wednesday for Facebook's Research program posing as a 14-year-old boy with two kids. He writes on Twitter that the process "required no proof of parental consent at all."

The tech giant's year of scandals and criticism does not seem to have impacted its bottom line, however. On Wednesday, Facebook reported $16.9 billion in revenue for the fourth quarter, along with growth in monthly active users and daily active users.