...In a post and a press release, the chief executive discussed Facebook Inc.’s initiatives to promote vaccines...For more than a month, Facebook researchers warned that comments on vaccine-related posts—often factual posts of the sort Facebook sought to promote—were filled with antivaccine rhetoric aimed at undermining their message, internal documents reviewed by The Wall Street Journal show. ...Users were seeing comments on vaccine-related posts 775 million times a day, the memo said, and Facebook researchers worried the large proportion of negative comments could influence perceptions of the vaccines’ safety. ...Despite Mr. Zuckerberg’s effort, a cadre of antivaccine activists flooded the network...