A young man calling himself Mohamed al-Alawi appeared in a YouTube video in August. He described himself as an investigative journalist in Egypt with a big scoop: The mother-in-law of Ukraine’s president had purchased a villa near Angelina Jolie’s in El Gouna, a resort town on the Red Sea. The story, it turned out, was not true. Ukraine denied it, and the owner of the villa refuted it. Also disconnected from reality: al-Alawi’s claim to being a journalist. Still, his story caromed through social media and news outlets from Egypt to Nigeria and ultimately to Russia – which, according to...