Four members of Congress wrote Facebook Tuesday demanding that the company stop giving advertisers the option of excluding by ethnic group.

The letter came in response to ProPublica’s story that Facebook allows advertisers to not show ads to those with an “ethnic affinity” for various minority groups. We bought an ad in Facebook’s housing categories that excluded those with an “affinity” for African-American, Asian-American, or Hispanic people.

“This is in direct violation of the Fair Housing Act of 1968, and it is our strong desire to see Facebook address this issue immediately,” wrote the lawmakers, who are members of the Congressional Black Caucus.

Under the Fair Housing Act, it’s illegal to “to make, print, or publish, or cause to be made, printed, or published any notice, statement, or advertisement, with respect to the sale or rental of a dwelling that indicates any preference, limitation, or discrimination based on race, color, religion, sex, handicap, familial status, or national origin.” Violators face tens of thousands of dollars in fines.

Facebook says it polices advertising on its platform. “If we learn of advertising on our platform that involves this kind of discrimination, we will take aggressive enforcement action,” the company wrote in a blog post this weekend.

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Facebook Lets Advertisers Exclude Users by Race

Facebook’s system allows advertisers to exclude black, Hispanic, and other “ethnic affinities” from seeing ads. Read the story.

“Facebook is complicit in promoting restrictive housing practices,” the lawmakers wrote. They called on Facebook founder and CEO Mark Zuckerberg to “remedy this matter swiftly and responsibly.”

In response to the letter, Facebook said in a statement: “We’ve heard from groups and policy makers who are concerned about some of the ways our targeting tools could be used by advertisers. We are listening and working to better understand these concerns.”

Like many social sites, Facebook offers advertisers the ability to target users by very specific interests or traits. That is particularly valuable for advertisers since it allows them to reach only those they think will be interested, and, conversely, to not spend money reaching those who won’t be.

Both Twitter and Google do not allow ad targeting by race.

Facebook has said that it has not been targeting by race, but rather simply to those with an “affinity” for various ethnic groups. “Affinity” for “African American,” “Asian American,” and “Hispanic” all appear under the Demographics category. Caucasian doesn’t – but it is in “additional interests.”