The most downloaded free app on both the App Store and Google Play for much of the last two months wasn’t TikTok, YouTube, or Instagram, but a shopping app that didn’t exist just four months ago. Temu offers steep discounts on a slew of products, mostly shipped directly from Chinese factories or warehouses. In addition to incredibly low prices, Temu can no doubt attribute its popularity to its strategy of giving free stuff to users who promote the app on their social networks and get friends and family to sign up. But the company—the U.S. offshoot of Chinese e-commerce giant...