TikTok in 2022 became the most-downloaded app in the world, quietly surpassing long-standing forebears Instagram and Twitter. By the end of this year, it will overtake YouTube as the social media platform users spend the most time watching. The video platform’s meteoric rise has astounded investors and industry experts. As it grows at breakneck speed, a new Guardian US series investigates some of the mounting questions surrounding its operations: the opaqueness of its algorithm, its effect on our brains, its ties to China, and its role in spreading election misinformation. “TikTok has become a source of culture, a reinforcement of culture, and a cauldron where culture is created,” said Emily Dreyfuss, a researcher at Harvard’s Shorenstein Center on Media, Politics and Public Policy. “It is time to take it seriously.” As we delve into the app that has reshaped the internet in just a few years, here’s what you need to know about its rise. Where TikTok came from TikTok as we know it was born from the marriage of two Chinese platforms – lip sync app Musical.ly and video platform Douyin. Musical.ly was founded in 2014 in Shanghai, and became widely popular in just a few years. In 2017, the platform was acquired by Chinese internet technology company ByteDance for $800m, allowing ByteDance to tap into Musical.ly’s 60 million users worldwide. Armed with Musical.ly’s US user base and its most popular features, ByteDance now boasted a broader international reach with its China-focused product, Douyin and global facing product, TikTok. How fast it has grown TikTok’s sprint towards world dominance began in earnest in 2018, when it first surpassed Facebook, Instagram, Snapchat and YouTube in downloads. The following year, it became the fourth most-downloaded non-gaming app in the world. In 2020, the Covid-19 pandemic poured gasoline on the app’s already-explosive growth. As billions of people struggled through repeated lockdowns, the app racked up downloads, reporting a 45% increase in monthly active users between July 2020 and July 2022. Today, TikTok boasts more than 1 billion active users, and the app is projected to surpass 1.8 billion users by the end of 2022. Combined, TikTok and Douyin make up the third largest social media platform by monthly users, behind Facebook and Instagram. Who uses it Fueling TikTok’s popularity is its relatively young user base. Nearly half of people between 18 and 30 in the US use the platform, a recent Pew Research Center report showed – and 67% of users between the ages of 13 and 18 use the app daily. According to that study, 16% of all teens use TikTok “almost constantly”. That makes TikTok the second-most popular app with young people, after YouTube which is used by 95% of teens. Instagram and Snapchat are used by approximately 60% of teens and Facebook by just 32%. TikTok’s powerful appeal to young people has made its competitors eager to mimic its rise. Companies such as Meta have struggled to stem a mass exodus of users under the age of 25 from their platforms, and internal documents leaked by Meta whistleblower Frances Haugen in 2021 showed that the company had made retaining young users at Instagram and Facebook a top priority. Meta in recent years has invested billions in Reels, its TikTok clone, but has struggled to monetize it. Growing concerns The algorithm: TikTok’s algorithm, which serves intensely fine-tuned content to users, is a key element of its success. The platform, according to internal documents leaked in 2021, optimizes content for minutes and hours of view time – a departure from its competitors who historically prioritized clicks and engagement. That key difference, and its algorithm’s stunning efficacy, have raised alarm at the mental health impact of such intensive targeting. Concerns about the consequences of extensive social media use have been long standing, with studies suggesting excessive use can exacerbate mental health problems and contribute to suicidal ideation. But TikTok comes with its own problems, including concerns that its algorithm-driven feed can make unhealthy trends go viral before they can be vetted for safety. Its ties to China: Meanwhile, US legislators are increasingly worried about the platform’s ties to China. In 2020, TikTok’s future in the US briefly looked insecure, when the Trump administration threatened to ban the platform outright or force ByteDance to sell it off to an American company. Under pressure, TikTok moved to store and process the data of US users to servers of American internet company Oracle. Starting in June 2022, TikTok said, “100% of US user traffic is being routed to Oracle Cloud Infrastructure”. But questions over Chinese access to US user data persist. TikTok has acknowledged that some backups of US user data would continue to be stored in China and Singapore. And reports from Buzzfeed this year showed US user data continued to be accessed by ByteDance employees in China. TikTok has reportedly been in talks with the US treasury department over a deal to address such national security concerns that would allow it to continue to operate in the US. Misinformation: Experts are also increasingly warning about the proliferation of misinformation on the platform, including during the war in Ukraine. One recent study on elections in Kenya found 130 videos on TikTok viewed more than 4m times featured election misinformation or hate speech related to the vote. As midterm elections approach in the US, all eyes are on TikTok for its biggest misinformation test yet: “This year is going to be much worse as we near the midterms,” said Olivia Little, a researcher who has studied misinformation at TikTok for research group Media Matters for America. “There has been an exponential increase in users, which only means there will be more misinformation TikTok needs to proactively work to stop or we risk facing another crisis.” TikTok has said it has taken several steps to support its users’ mental health, including introducing new tools that “promote kindness” on the app such as filters that allow users to more easily filter offensive comments and an automatic pop-up prompt that asks users “to reconsider” leaving comments that violate guidelines. The company has said US legislators’ national security concerns are overblown, arguing that the platform does not share user data with the Chinese government and that it uses “a combination of technology and thousands of safety professionals” to identify and remove videos that violate its policies, including extremist content. To combat the spread of election misinformation on its platform, the company said, it provides accurate election information and partners with independent fact-checking agencies to help direct its content moderation policies around elections. According to its transparency reports, it removed 350,000 videos related to election misinformation in the latter half of 2020.