The most downloaded app on the App Store for the last year makes almost no money, is barely understood by anyone over 25, and has already faced investigations, fines and bans on three continents.

TikTok’s success has taken regulators, parents and its competitors by surprise. But with the Information Commissioner’s Office (ICO) in the UK now investigating the company over its handling of young users’ private data, can reality catch up with the viral smash?

At its core, TikTok is a video-sharing app. Users film themselves in 15-second clips, typically set to music, and upload them to be viewed by followers and strangers alike. If it seems like it came out of nowhere, that’s because in part, it did: the app as it is today is a merger of the original TikTok, which was launched internationally in September 2017, and the earlier viral sensation Musical.ly.

The latter had already become one of the most popular social media platforms for UK and US teenagers by the time it was purchased by TikTok’s owner, ByteDance, in November 2017, and its influence still pervades the platform to this day. More importantly, the merger meant TikTok ended up on the smartphones of more than 60 million users overnight.

ByteDance had experience of that sort of scale, however. The Chinese startup was already famous in its homeland for viral news app Toutiao, which hit 120 million daily users in 2017, as well as Douyin, the China-only version of TikTok that has been kept separate to comply with Beijing’s strict censorship regime.

That pre-existing scale also gave TikTok another weapon: a huge war chest. Before ByteDance was even earning revenue from the app, the company was the single largest advertiser on Snapchat, spending nearly $1bn (£800m) on app install ads on the messaging platform, according to the Wall Street Journal. That spending was recreated on Facebook, Instagram, poster campaigns and TV adverts.

As a result, according to analytics firm SensorTower, TikTok has been the No 1 app on the worldwide App Store for five consecutive quarters, with an estimated 500 million users worldwide. Even if TikTok’s spending dies off, it now has the momentum to survive, according to Emma Worth of the marketing firm Ralph Creative.

“The app itself offers something no other app does and that’s why it has become so successful: paid acquisitions just help more people know about it,” she said. “Young people are fed up with the narcissistic influencer movement on other channels, seeing ‘the perfect life’, ‘the perfect body’, the ‘perfect relationship’ and that’s why they’ve moved to TikTok.”

But with increased scale has come increased scrutiny. In February, the Federal Trade Commission (FTC) in the US fined the company $5.7m) £4.2m for collecting the personal data of children under 13 without parental consent. In April, TikTok was banned in India, where it claims to have more than 120 million monthly active users, over concerns that the app was being used to share sexually explicit material. The ban was lifted a week later.

The ICO is investigating because of the same data protection concerns as the FTC, but with additional focus on the controls available on the app’s direct messages. The UK watchdog fears that adults are able to send private messages to children they do not know.

It is the ability to live stream that worries the NSPCC and others.

“We know that a worrying number of children are being contacted via livestreaming apps, such as TikTok, by adults who use them as a gateway to groom and harm young people,” said a spokesperson for the charity. “Our research found that that a quarter of 40,000 children (seven- to 16-year-olds) have livestreamed online with someone they have never met, and one in 20 children had been asked to take their clothes off.”

The NSPCC’s research has been backed up by other investigations. Young, female users of TikTok regularly report creepy or intrusive replies to their videos, according to BuzzFeed news site, and bemoan the failure of ByteDance to respond.

The problem is worsened by the nature of the app: rather than prioritising the videos of people you follow, it instead guides users to the fully algorithmic “for you” feed. “If some creepy guy just keeps liking videos of younger girls doing similar audios or soundtracks or hashtags, those are going to keep coming up on his ‘for you’ page,” one user told BuzzFeed.

While the lack of moderation is concerning some users, particularly when it comes to child safety, it is the opposite that concerns others. As a Chinese startup, ByteDance has a very different conception of what it means to be a social media company than the Californian tech firms that have dominated the field until now.

Worth said TikTok may be being unfairly singled out due to the lack of adults on the platform. It is no worse than other apps, but adults just do not “get” like they did Instagram or Snapchat, she continued. “Other social media channels are riddled with porn and indecent images and all open up a world of the unknown and risky. Tiktok is feeling the hit because it’s attracted a younger audience.

“I think because of its raging success, people who don’t ‘get it’ or don’t use it, are more inclined to want to take it down.”