Facebook says it is not dead. Facebook also wants you to know that it is not just for “old people,” as young people have been saying for years. Now, with the biggest thorn in its side — TikTok — facing heightened government scrutiny amid growing tensions between the U.S. and China, Facebook could, perhaps, position itself as a viable, domestic-bred alternative. There’s just one problem: young adults like Devin Walsh have moved on. “I don’t even remember the last time I logged in. It must have been years ago,” said Walsh, 24, who lives in Manhattan and works in public...