Algorithmic political bubbles are hard to detect because they show something different to each person. Only by comparing notes can people map the boundaries of what a platform chooses to show its users. Doing so, when legal, allows independent researchers to detect discrimination and hold platforms accountable for their actions. To find out if Facebook's rainbow Pride reaction was a case of digital gerrymandering, our three-person team—a data scientist, a survey researcher, and an ethnographer of youth social-media practices—conducted an algorithmic audit, asking hundreds of Facebook users in 30 cities to report if they could access the Pride reaction.

Our audit asked two questions. First, are there U.S. cities where everyone is allowed to give a rainbow reaction? Second, do Facebook’s own LGBTQ-interest algorithms predict who has access elsewhere?

By using Facebook’s algorithms, we based our audit on the way that Facebook’s software sees the world. When advertisers publish an ad with Facebook, the company asks them to define the regions, interests, and demographics of the people they want to reach. While the platform’s gender targeting does not allow grouping by LGBTQ identities, their algorithms do infer LGBTQ-interest based on what people like, share, and write about. People can be categorized for their interests in, for instance, “Gay Pride,” “LGBT Culture,” “Pride Parade,” “Rainbow Flag (LGBT),” and “LGBT Social Movements.” Since Facebook allows advertisers to include or exclude people from those categories, we could survey people to discover if LGBTQ-interested people have a different experience on the platform from people that Facebook categorizes as not LGBTQ-interested.

Across 15 states that are home to the largest U.S. cities, we chose a large city per state and paired it with a smaller city elsewhere in the state. Within each city, we used Facebook’s ad targeting to recruit people who the platform’s algorithms think are interested in LGBTQ issues, compared to people who aren’t. We then tested the correlation between LGBTQ interest and access to the Pride reaction.

Among the cities we investigated, an overwhelming percentage of people without LGBTQ interests reported having the pride emoji in New York City, San Francisco, Chicago, Seattle, and Boston. Yet many other cities among the largest 25 in the U.S. were excluded from city-wide access, including Philadelphia, Detroit, Phoenix, and Nashville.

In places without city-wide access, Facebook’s LGBTQ advertising groups correlated strongly with people's ability to use the rainbow reaction. On average, people with LGBTQ interests who responded to our ads were 46 percent more likely than the non-LGBTQ interest group to report having access to the rainbow reaction. It's possible that people in the LGBTQ interest groups received the rainbow because they chose to “like” the LGBTQ@Facebook group, which the company says will unlock the rainbow reaction.

Kristina Boerger, a 52-year-old musician and human-rights organizer from Greencastle, Indiana, was surprised that other people could use the reaction but not her. “It certainly wouldn’t be because Facebook doesn’t know that I am queer,” she said. “That would be one of the first things they know about me.”