he number of people using X daily is falling, more than a year after tech billionaire Elon Musk bought the app formerly known as Twitter. Data from two research firms and figures published by Musk and X suggest a deteriorating situation for X by some metrics. Musk has marketed it as the world’s “town square,” but in number of users it continues to lag far behind social media rivals that focus on video, such as Instagram and TikTok. In February, X had 27 million daily active users of its mobile app in the U.S., down 18% from a year earlier,...