It has surfaced in the digital correspondence of Arianna Huffington and Nora Ephron. Wendy Williams, the talk-show host, says she wishes she could stop using it, but can’t. Anne-Marie Slaughter doesn’t use it, but knows several professional women who do. And in Diane Sawyer’s newsroom, staffers say, the anchor XOs so frequently that an XO omission can spark a major panic.