WASHINGTON — Days before the presidential election, James Alefantis, owner of a local pizza restaurant called Comet Ping Pong, noticed an unusual spike in the number of his Instagram followers. Within hours, menacing messages like “we’re on to you” began appearing in his Instagram feed. In the ensuing days, hundreds of death threats — one read “I will kill you personally” — started arriving via texts, Facebook and Twitter. All of them alleged something that made Mr. Alefantis’s jaw drop: that Comet Ping Pong was the home base of a child abuse ring led by Hillary Clinton and her campaign...