Nathan Apodaca never expected to propel Fleetwood Mac’s “Dreams” back onto the Billboard Hot 100. But that’s exactly what happened last September, after the 38-year-old posted a video of himself skateboarding and drinking cran-raspberry juice to the tune of the 1977 hit on TikTok, where it went viral. Such is the power of America’s most popular app, where some 50 million U.S. users spend an average of 46 minutes every day. Its secret: an uncannily effective discovery algorithm that surfaces a never-ending stream of videos. Now TikTok—which infamously drew the ire of former President Trump over questions about user-data storage—is hosting livestream events (like a Justin Bieber concert) and partnering with Shopify to offer more in-app purchases, signs that its ambitions are much grander than helping users waste time.