Imagine if, during the Jim Crow era, a newspaper offered advertisers the option of placing ads only in copies that went to white readers.

That’s basically what Facebook is doing nowadays.

The ubiquitous social network not only allows advertisers to target users by their interests or background, it also gives advertisers the ability to exclude specific groups it calls “Ethnic Affinities.” Ads that exclude people based on race, gender and other sensitive factors are prohibited by federal law in housing and employment.