Facebook employees have warned for years that as the company raced to become a global service it was failing to police abusive content in countries where such speech was likely to cause the most harm, according to interviews with five former employees and internal company documents viewed by Reuters.

For over a decade, Facebook has pushed to become the world's dominant online platform. It currently operates in more than 190 countries and boasts more than 2.8 billion monthly users who post content in more than 160 languages.

But its efforts to...