Facebook just started promoting what on first glance might seem like an ordinary VPN app that can keep your data private as you browse the Web. But, there are reasons to be wary of Onavo Protect and question Facebook's intentions to push it inside its main app.

To see what we're talking about, head over to the Facebook iOS app, press the navigation button (three horizontal lines on the bottom right), scroll down, and you may see a new option labeled "Protect." Clicking this option will redirect you to the App Store where you can download Onavo Protect, a VPN app made by the Israeli startup Onavo, which Facebook acquired in 2013.

Here's what might irk you: Onavo Protect doesn't just help 'keep you and your data safe,' as it promises; it also tracks you, and sends information about the apps and websites you use back to Facebook.

"Onavo collects your mobile data traffic," the app's description says. "This helps us improve and operate the Onavo service by analyzing your use of websites, apps and data. Because we're part of Facebook, we also use this info to improve Facebook products and services, gain insights into the products and services people value, and build better experiences."

The app promises to warn you when you visit potentially malicious or harmful sites and help secure your personal information when you're connected to a public Wi-Fi network. But, without reading through the app's description, you would likely be unaware that Onavo Protect also sends your mobile browsing history to Facebook.

That data gives Facebook insights about "apps that are becoming breakout hits… which are seeing slowing user growth… which apps' new features appear to be resonating with their users, and much more," according to TechCrunch, which first reported on Facebook's move to promote Onavo Protect in its main app. The report notes that Facebook has already leveraged data from Onavo Protect "in a number of ways, most notably in its battle with Snapchat."

CNBC, meanwhile, pointed out that Facebook is peddling this app "without first disclosing that it's owned by Facebook."