Civil society groups in Myanmar... criticized Facebook’s chief executive, Mark Zuckerberg, arguing that he mischaracterized his company’s effectiveness at detecting and quashing messages encouraging violence in the country... Taking aim at comments Mr. Zuckerberg made in a recent interview, the groups said that Facebook had no consistent methods for dealing with hate speech in Myanmar... In a conversation with Ezra Klein of Vox this week, Mr. Zuckerberg referred to a pair of chain letters that were widely shared in Myanmar on Facebook Messenger last year. One message warned Buddhist groups about an imminent attack by Muslims, expected on Sept. 11. The other, spread among Muslims, cautioned of violence from Buddhist nationalists on the same date... In an open letter, the organizations, which have worked with Facebook to flag dangerous and misleading content, including the chain letters, said the messages had spread for days, caused widespread fear and set off at least three violent incidents... Facebook responded to the criticism... “We are sorry that Mark did not make clearer that it was the civil society groups in Myanmar who first reported these messages,” it said. “We took their reports very seriously and immediately investigated ways to help prevent the spread of this content. We should have been faster, and are working hard to improve our technology and tools to detect and prevent abusive, hateful or false content.”