With all the data that comes from 2 billion+ monthly users, Facebook can let advertisers target people based on virtually any characteristic. An algorithm automatically generates these categories based on words and phrases found in profiles, such as “gym lover,” but a ProPublica report found that the system was also allowing companies to target their ads toward anti-Semites.

Some of the categories available for ad-buyers included “Jew Hater,” “How to burn Jews,” “Hitler did nothing wrong,” and “History of ‘why Jews burned the world.’” Most of these were described as “interests” or “fields of study,” and some terms even autocompleted themselves when reporters typed “jews h” into the advertising category search box.

ProPublica was told that as the number of people who fall into the groups was so small – 2300 users in the “Jew Hater” section – it needed to add more categories to purchase ads. Terms such as “Nazi party” were added, and Facebook’s system suggested adding “second amendment,” presumably as it associated gun enthusiasts with anti-Semites. Once the potential audience was large enough, the algorithm approved the ads.

Facebook has since removed the offending categories. "Our community standards strictly prohibit attacking people based on their protected characteristics, including religion, and we prohibit advertisers from discriminating against people based on religion and other attributes," said Rob Leathern, product management director at Facebook. "However, there are times where content is surfaced on our platform that violates our standards.”

"We know we have more work to do, so we're also building new guardrails in our product and review processes to prevent other issues like this from happening in the future," he added.

Despite Facebook’s claims that it would stop similar categories from appearing, a follow-up investigation by Slate found it could use terms such as “Kill Muslimic Radicals” and “Ku-Klux-Klan” for advertising purposes.

The news comes just a week after Facebook admitted it sold $100,000 worth of ads to a Russian “troll farm.”