TikTok is thought to have about half a billion active users worldwide, with a large percentage in their teens or early 20s.
The short-form video app has become a fertile birthing ground for memes, many of them music-related.
One example involved users and their pets switching into cowboy and cowgirl clothing at a certain point in rapper Lil Nas X's song Old Town Road - something that is credited with it becoming a huge hit.
But some politicians are worried the app's Chinese owner, Bytedance, poses a risk to national security.
Regulators have also raised their own safety concerns.
So is the app safe or is the fervour about TikTok a tempest in a teapot?
How does it work?
People - mostly under-20s - use the app to post 15-second videos. Many involve lip-synching to songs, comedy routines and/or unusual editing tricks.
These are then made available to both followers and strangers. By default, all accounts are public, although users can restrict uploads to an approved list of contacts.
An algorithm analyses what type of material each user pays most attention to, to hook them on other clips, and it's easy to lose track of time as one auto-plays after another. Members can also search for specific topics or users and browse by clicking on hashtags.
TikTok also allows private messages to be sent but this facility is limited to "friends".
Anyone over the age of 13 can use it and there are parental controls.
Many of its long-term members originally downloaded Musical.ly, a rival short-form video app owned by a separate Chinese start-up. But it was acquired by Bytedance, in 2017, which merged the two platforms.
Beijing-based Bytedance, also has a sister app, Douyin. This is run on a different network in order to comply with Chinese censorship rules.
The company is no stranger to controversy. This past year, it garnered a temporary ban in India, a US counter-intelligence investigation and a record £4.3m fine after Musical.ly was found to have knowingly hosted content published by under-age users.
Why are people worried about TikTok's handling of data?
Critics in the US warn the app has the potential to compromise users' privacy.
It’s trendy to pretend that concerns about Chinese tech domination are just a smoke screen by US executives, but if you actually care about privacy, safety and content moderation then you need to pay attention to TikTok. https://t.co/wg5TDoJN4z— Alex Stamos (@alexstamos) November 5, 2019
Alex Stamos, who is the former chief security officer of Facebook and now a Stanford professor, tweeted his concern after reports emerged of a clash between Bytedance's US and Chinese employees.
Former members of the US content-vetting team - which removes clips featuring terrorism and pornography - had said their China-based colleagues had also told them to take down videos that would not normally be flagged for abusive content in the US, according to a report in the Washington Post.
The deleted videos reportedly contained heavy kissing, suggestive dance moves and political debate.
Others worry about the fact China requires its social-media apps to provide the state access to users' information.
But Bytedance says data about users in other countries is stored separately and not shared with the Chinese authorities.
Why else are US lawmakers concerned?
Earlier this week, US politicians asked TikTok to testify at a congressional hearing. They wanted the company to clear up allegations it was beholden to the Chinese state.
Several US senators have joined the call for a government investigation.
Arkansas senator Tom Cotton has claimed TikTok might be the target of a foreign-influence campaign, like those carried out on Facebook and Twitter in the 2016 election.
Bytedance says TikTok does not carry political ads - but the senator said there were still concerns China might find other ways to use "personal sensitive information" about the app's users.
.@SenSchumer and I are concerned about the risk Chinese-owned @tiktok_us may pose to our national security. We requested the Acting Director of National Intelligence conduct an assessment of this platform and brief Congress on his findings. https://t.co/tGmt5ysWza— Tom Cotton (@SenTomCotton) October 24, 2019
And he worries the company could be compelled to co-operate with intelligence work driven by the Chinese Communist Party.
What is TikTok doing to allay concerns?
The company says it has changed over the course of 2019.
It used to apply "one-size-fits-all" content guidelines to all its international markets - but now US moderation decisions are handled locally.
Vanessa Pappas, US general manager for TikTok told BBC News: "Consistent with our rapid growth, we are working to further strengthen the capabilities and increase the autonomy of the US team."
In addition, the company has hired a third party to carry out an audit meant to ensure users' data is not transmitted to China via third-party apps that can plug into TikTok.
What does this mean for TikTok?
Grindr, the gay dating app acquired by the Chinese company Kulun, represents a cautionary tale for TikTok.
The Committee on Foreign Investment in the United States (Cifius) has compelled Kulun to sell Grindr, which it has agreed to do in 2020.
Cifius has the power to unwind foreign takeovers of US companies if it finds there to be a national security threat.
And it ruled Grindr held too much personal information about US soldiers.
The same committee is now reported to be looking into the takeover of Musical.ly on the basis the company had a California base, even though it was headquartered in Shanghai.
If Bytedance is unable to convince lawmakers - who are in the midst of an election cycle - it does not pose a potential threat, TikTok could suffer a similar fate.
Three things could happen:
- the app could be geo-blocked in the US, where it has 26.5 million monthly active users
- TikTok could be spun off as a separate company based outside China
- Bytedance might have to sell TikTok to another technology company
TikTok also faces regulatory scrutiny in the UK.
Information commissioner Elizabeth Denham confirmed in July she was investigating how it used children's data.
"We do have an active investigation into TikTok right now, so you can watch that space," the regulator told MPs.