By now you've probably gathered that Facebook uses things like your interest, age and other demographic and geographic information to help advertisers reach you. Then there's the stuff your friends do and like — the idea being that it's a good indicator for what you might do and like. So, if you have a friend who has liked the New Yorker's Facebook page, you might see ads for the magazine on your Facebook feed.
But that's just the tip of the iceberg. Facebook and advertisers can also infer stuff about you based on things you share willingly. For example, Facebook categorizes users into an "ethnic affinity" based on what it thinks might be their ethnicity or ethnic influence. It might guess this through TV shows or music you've liked. Often, Facebook is wrong — and while it's possible to remove it, you can't change it. There is also no "ethnic affinity" option for whites.
While there are plenty of good reasons advertisers may want to target people of a particular ethnicity, this became a problem for Facebook in 2016, when ProPublica found that it let advertisers exclude specific ethnic groups from seeing their ads. When it comes to housing and employment ads, this is illegal.
In late 2017, Facebook said it was temporarily blocking advertisers' ability to target based on ethnic affinity, along with other things such as religious or LGBT affinity. Advertisers can still target those groups — just not exclude them. Facebook, which said it is conducting an audit of how the feature can be misused, did not say when it would lift the block.
While some advertisers want to reach large swaths of people, others like more specific targeting. As Facebook explains in a guide for advertisers, it's possible to refine an ad's audience on things like what people post on their timelines, apps they use, ads they click, demographics such as age, gender and location, and even the mobile device they use or their network connection. Based on this information, advertisers can either include or exclude categories such as homeowners, "trendy moms," people who moved recently, conservatives, or people interested in cooking, for example.
That said, Facebook warns advertisers not to narrow their audience too much by being overly specific, which can make the ads less effective — since fewer people will see them.