Mark Zuckerberg has already made it clear he has no love for TikToK, but we now have an even clearer look at just why he's so concerned about the Chinese rival. In short: the app is growing like crazy, even by Facebook standards.

TikTok was downloaded more than 738 million times in 2019, according to analytics company Sensor Tower, beating out both Instagram and Facebook. In fact, there was only one app that was downloaded more, according to the firm: the Facebook-owned WhatsApp.

But while a Facebook property may hold the top spot, the report underscores just how big of a threat TikTok is to Facebook's dominance.

"TikTok’s 2019 installs represent 44 percent of the 1.65 billion downloads that the app has had to date," Sensor Tower wrote in a blog post.

Of course, not everyone who downloads the app will go on to become an active user. But the numbers do tell the story of an app experiencing meteoric growth, especially in China, where Facebook isn't allowed to operate. China, where the app is known as Duoyin, was ByteDance's second largest market after the U.S., Sensor Tower notes.

All that makes TikTok an existential threat to Facebook, which is under constant pressure to maintain user growth across its services. And though Facebook has sought to copy some of TikTok's features in its own apps, those efforts have yet to take off. (Mark Zuckerberg also reportedly tried and failed to buy Musical.ly prior to ByteDance's 2017 acquisition of the app that would later become TikTok.)

Meanwhile, Zuckerberg has gone out of his way to say that TikTok, which is owned by Chinese company ByteDance, deserves more scrutiny.

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"While our services, like WhatsApp, are used by protesters and activists everywhere due to strong encryption and privacy protections, on TikTok, the Chinese app growing quickly around the world, mentions of these protests are censored, even in the U.S.," Zuckerberg said during a speech at Georgetown University last year. "Is that the internet we want?"

China, he warned, "is now exporting their vision of the internet to other countries." (TikTok has denied that the Chinese government has influence over the app's policies and ByteDance has reportedly considered selling at least part of the company in response to concerns from U.S. officials.)

But whether or not you think Zuck's paranoia of the Chinese government he once wooed is in good faith or not, seeing TikTok grow faster than Facebook has got to sting at least a little.