A murderous anti-Semitic theme appeared on Facebook Sunday, when a user named “Alex Cookson” launched an open invitation to an “event” called “Kill a Jew Day.” The page on the popular social networking Web site urged users to violence “anywhere you see a Jew” between July 4 and July 22. A large image of a swastika was placed at the top of the page. Under the heading “description,” Cookson wrote, “You know the drill guys.” It was the fourth time that a call to murder Jews had been put on Facebook within recent days.