U.S. exchanges and Wall Street’s top cop on Thursday warned about a heightened threat of fraud mostly involving the initial public offerings of small companies, driven in part by a social media-driven pump-and-dump scheme called “pig butchering.” In a rare move, Nasdaq Inc, Intercontinental Exchange Inc’s New York Stock Exchange and the Financial Industry Regulatory Authority (FINRA) published notices about manipulation and fraud tied to market debuts. Some investors harmed by the pump-and-dump schemes appear to be victims of an evolving social media scam called “pig butchering,” Finra said. “These schemes sometimes begin with a seemingly misdirected text message or...