On the night Shelby Renae first went viral on TikTok, she felt so giddy she could barely sleep. She'd spent the evening painting her nails, refreshing her phone between each finger - 20,000 views; 40,000 - and by the next morning, after her video crossed 3 million views, she decided it had changed her life.

She didn't really understand why it had done so well. The 16-second clip of her playing the video game "Fortnite" was funny, she thought - but not, like, millions-of-views funny. She wasn't a celebrity: She grew up in Idaho; her last job was at a pizza shop. But this was just how the world's most popular app worked. TikTok's algorithm had made her a star.

Now 25, she spends her days making TikTok videos from her apartment in Los Angeles, negotiating advertising deals and always chasing the next big hit. Many days, she feels drained - by the endless scramble for new content; by the weird mysteries of TikTok's algorithm; by the stalkers, harassers and trolls. Yet still, in her off hours, she does what all her friends do: watches TikTok. "It will suck you in for hours," she said.

If you have not used TikTok, you are rapidly becoming the global exception. In five years, the app, once written off as a silly dance-video fad, has become one of the most prominent, discussed, distrusted, technically sophisticated and geopolitically complicated juggernauts on the internet - a phenomenon that has secured an unrivaled grasp on culture and everyday life and intensified the conflict between the world's biggest superpowers.

Its dominance, as estimated by the internet firms Cloudflare, Data.ai and Sensor Tower, is hard to overstate. TikTok's website was visited last year more often than Google. No app has grown faster past a billion users, and more than 100 million of them are in the United States, roughly a third of the country. The average American viewer watches TikTok for 80 minutes a day - more than the time spent on Facebook and Instagram, combined.

Two-thirds of American teens use the app, and 1 in 6 say they watch it "almost constantly," a Pew Research Center survey in August found; usage of Facebook among the same group has been cut in half since 2015. A report this summer by the parental-control tool Qustodio found that TikTok was both the most-used social media app for children and the one parents were most likely to block. And while half of TikTok's U.S. audience is younger than 25, the app is winning grown-ups' attention, too; the industry analyst eMarketer expects its over-65 audience will increase this year by nearly 15 percent. (AARP last year even unveiled a how-to guide.)

More than just a hit, TikTok has blown up the model of what a social network can be. Silicon Valley taught the world a style of online connectivity built on hand-chosen interests and friendships. TikTok doesn't care about those. Instead, it unravels for viewers an endless line of videos selected by its algorithm, then learns a viewer's tastes with every second they watch, pause or scroll. You don't tell TikTok what you want to see. It tells you. And the internet can't get enough.

"We're not talking about a dance app," said Abbie Richards, a researcher who studies disinformation on TikTok, where she has half a million followers. "We're talking about a platform that's shaping how a whole generation is learning to perceive the world."

TikTok's cultural influence on a new generation of media has led to some astounding ripple effects. Viral videos of people delighting in their favorite books, many of them with the hashtag #BookTok, which has 78 billion views, helped make 2021 one of the publishing industry's best sales years ever. Books from the author Colleen Hoover, BookTok's biggest star, have sold more copies this year than the Bible, according to data from NPD BookScan, which tracks sales at 16,000 stores nationwide.

America's biggest technology innovators are reinventing themselves in TikTok's image, not only in developing short-video copycats - Meta's Reels, YouTube's Shorts - but in swapping out networks of friends and families for feeds of strangers chasing viral glory. TikTok's model could soon shape the entire internet.

But TikTok's ownership, by the Beijing-based tech giant ByteDance, has also made it one of the biggest pariahs in Washington. Former president Donald Trump tried to dismantle it. Top branches of the U.S. government and military have banned it from government-issued phones. And members of Congress insist it could be a Trojan horse for a secret Chinese propaganda and surveillance machine.

Even as the app has transformed into a public square for news and conversation, TikTok's opaque systems of promotion and suppression fuel worries that China's aggressive model of internet control could warp what appears there.

TikTok executives have argued they aren't influenced by government agendas and want only to foster an entertainment platform that is fun and conflict-free. They have worked to soothe doubts and make friends in a hostile Washington by hiring U.S.-based specialists, promising transparency and piping American users' data through servers in the United States.

But former TikTok employees and technical experts argue that the company's fixes do nothing to address its biggest risk: that its top decision-makers work in a country skilled at using the web to spread propaganda, surveil the public, gain influence and squash dissent. That crisis of trust has led to an ongoing debate among U.S. regulators: whether to more closely monitor the app or ban it outright.

Many TikTok creators say speculation about the app's Chinese roots distracts from the more grounded issues they face as a result of its explosive growth. TikTok's ability to make anyone go viral overnight, they say, has meant that the anger and pressure once endured mostly by big influencers have become facts of life for the masses.

Drew Maxey, a high school literature teacher in St. Louis, said he has gotten used to seeing glimpses of TikTok in class and hearing its sounds in the school hallways. It has become the main way most students socialize and pass the time; he's even become a TikToker, gaining more than 50,000 followers with videos that use comic books as literary tools.

But he worries about how TikTok's enigmatic machinery and students' desire for viral attention have already shaped how some of them talk and behave. He's started changing his wording, too; on some book videos, he won't even say the word "death," anxious it might stunt his reach.

"Everything they need, they get from TikTok," he said. "Yet we're training a whole generation of people not to say what they actually mean."

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TikTok starts studying its users from the moment they first open the app. It shows them a single, full-screen, infinitely looping video, then gauges how they react: a second of viewing or hesitation indicates interest; a swipe suggests a desire for something else. With every data point, TikTok's algorithm narrows from a shapeless mass of content to a refined, irresistible feed. It is the ultimate video channel, and this is its one program.

The "For You" algorithm, as TikTok calls it, gradually builds profiles of users' tastes not from what they choose but how they behave. While Facebook and other social networks rely on their users to define themselves by typing in their interests or following famous people, TikTok watches and learns, tapping into trends and desires their users might not identify.

TikTok fans say they've been both surprised and unsettled by an algorithm that can read them eerily well, showing them videos they never searched for or even realized they wanted to see.

TikTok tells advertisers that these "continuous cycles of engagement" make it more memorable, emotional and immersive than TV. A company-funded study that used brain-imaging scans on test subjects found that TikTok users engaged with the app about 10 times a minute, twice as often as its social media peers.

TikTokers are increasingly using the app as a visual search tool; 40 percent of Generation Z respondents to a Google survey this year said they had opened TikTok or Instagram, not Google, when searching for nearby lunch spots. (One tweet in June, "I don't Google anymore I TikTok," has been 'liked' 120,000 times.)

And as Americans' trust in news organizations has fallen, TikTok's role as a news source has climbed. One in three TikTok viewers in the United States said they regularly use it to learn about current events, Pew Research Center said last month. In the United Kingdom, it's the fastest-growing news source for adults. (The Washington Post's TikTok account has more than a million followers.)

After cornering the market on entertainment, TikTok began offering its model of behavioral tracking and algorithmic suggestion to advertisers, promising them a way to know which ads people find most compelling without having to ask. It was an instant hit: The company's ad revenue tripled this year, to $12 billion, according to eMarketer estimates, and is expected to eclipse YouTube at nearly $25 billion by 2025. In the United States, the cost to advertisers for TikTok's premium real estate - the first commercial break a viewer sees in their feed, known as a "TopView" - has jumped to $3 million a day.

Beyond traditional marketing, TikTok has rapidly industrialized the way companies pay young people to hawk their stuff. TikTok runs a giant catalogue of people, the Creator Marketplace, that companies can use to sort creators by their interests and follower counts; the service is invite-only, and creators have to post frequently if they want the chance to get paid. Influencers paid to promote goods in their videos now make more ad money on TikTok than Facebook: roughly $750 million, U.S. estimates from Insider Intelligence show. (Instagram, which beats both of them, this summer debuted its own "Creator Marketplace" clone.)

At a time when Silicon Valley's stock prices are crumbling, TikTok's success has triggered deep jealousy - especially for Facebook, which in February reported it had lost users for the first time in its 18-year history. (The top link on all of Facebook in the second quarter of this year was TikTok, Facebook's parent company Meta said.)

Meta tried beating TikTok by hiring a Republican lobbying firm to undertake a secretive letter-writing and lobbying campaign calling it the "real threat" to America's teens. But by the summer, Meta ended up just copying TikTok's style, ditching its focus on people's friends and families and swapping in computer-selected unknowns.

TikTok, however, seems bent on taking on a wider range of digital life. It's tested features for interactive minigames and job résumés. It started selling concert tickets. It built a live-streaming business used for meal-cooking showcases, lottery scratch-offs, tarot readings and apartment tours. And it tested a shopping feature that would let viewers buy products from QVC-style live streams in a few quick taps.

Even without that expansion, there can be no denying that TikTok has become a world-shaping force of its own - so colorful and compelling that many viewers find it hard to quit. That's even the case in Russia, where the company, abiding by Kremlin directives, has blocked everyday Russians from posting new TikToks or seeing any videos from outside the country since the Russian military invaded Ukraine.

The TikTok people watch in Russia has become its own parallel universe, frozen in time - an endless stream of old Russian videos and pro-Kremlin propaganda. But many young Russians continue to use it "quite actively" nearly eight months into the war, said a few who spoke with The Post on the condition of anonymity because of the country's draconian speech laws.

Some teens said they use technical workarounds to see foreign TikToks, risking punishment for a glimpse of the outside world. But one 18-year-old said he just settles for watching whatever the algorithm shows. "Yes, all videos are old," he said. "But it's still enough."

The Washington Post's Will Oremus and Natalia Abbakumova contributed to this report.