It is the Chinese-owned short video-sharing site which you may not have heard of if you are over 30.
But on Tech Tent this week we explore why TikTok could be the biggest threat to Facebook.
We learned two important things about TikTok this week - it's earning a surprising amount of money, and Mark Zuckerberg sees it as a serious rival.
Figures obtained by the Reuters news agency showed that its owner ByteDance earned revenues of between $7bn (£5.7bn) and $8.4bn in the first half of the year. And while it made losses over the period as a whole, it moved into profit in June and is confident of continuing profitability in the second half of the year.
At a time when the likes of WeWork, Uber and Lyft are warning that the path to profits is long and uncertain, that is an impressive performance from a tech business just a couple of years old.
And Mark Zuckerberg seems to agree. A leaked recording of a Facebook staff meeting obtained by tech news site The Verge shows him telling his team just how rapidly TikTok is growing.
"It's the first consumer internet product built by one of the Chinese tech giants that is doing quite well around the world. It's starting to do well in the US, especially with young folks. It's growing really quickly in India. I think it's past Instagram now in India in terms of scale. So yeah, it's a very interesting phenomenon."
He then goes on to outline plans to take on this "interesting phenomenon" by launching a copycat version in Mexico, where TikTok has yet to make much of an impact.
So how big is TikTok among teenagers?
I got some insight into its growth when I gave a talk in a South Wales school a few weeks ago. As is my habit, I started with an instant poll of the audience's social media habits. Although Facebook was on the wane, a forest of hands went up for Instagram and Snapchat. But while at a similar talk some months earlier TikTok hadn't got a mention, this time there were plenty of nods and smiles from the audience.
And the research firm AppAnnie, which monitors downloads of mobile applications, reinforces that anecdotal evidence. Its European analyst Paul Barnes tells us that outside China, Android users spent 1.1 billion hours on TikTok in August, a 400% increase on a year ago.
TikTok has been accused of censoring posts that mention the Tiananmen protests
But he says that what should really worry other social media firms is the level of commitment of TikTok users, compared to someone posting a picture on Instagram or updating their Facebook status.
"You have to put quite a lot of time in to make a good TikTok video," he explains. "So if I was a rival and looking at it, I'd be quite jealous of the amount of time and the amount of commitment that users of TikTok are putting into publishing their own content on there."
But with this rapid growth come concerns about how safe the platform is. Earlier this year a BBC investigation found TikTok failing to close the accounts of some adult users who had contacted children with sexual messages. TikTok says "promoting a safe and positive app environment for our users is our top priority".
It is also clear that it is very much a Chinese company, with all that entails in terms of censorship. The Guardian recently published a leak of internal documents showing that moderators are instructed to delete videos mentioning sensitive subjects such as Tiananmen Square. There are also suspicions that discussion of the unrest in Hong Kong has been censored.
But it is doubtful that its millions of users in Europe and the United States either know or care that TikTok is a Chinese company. When we stopped young people outside a London fashion store to get their views on it they described it as "fun", "catchy", "funny", and "cringey but a massive trend - it's everywhere".
Perhaps not the kind of words teenagers would use these days to describe either Facebook or Instagram - and that is why Mark Zuckerberg is right to be worried.