The organizers of the #StopHateforProfit Facebook ad boycott have written a letter to the House Judiciary Committee asking the members to particularly press Facebook CEO Mark Zuckerberg about the company’s alleged monopoly over the advertising sphere.

First reported by Axios, the letter suggests several pointed questions that lawmakers could ask: For instance, what percentage of U.S. digital ad spending runs through Facebook and its subsidiaries, what this means for small and medium businesses, and whether there are any alternatives for advertisers to reach certain demographics with the power and efficiency that Facebook uses. The questions seem intended to get at whether Facebook is truly the monopoly it claims not to be.

In June, several hundred major brands, including Coca-Cola, Unilever, and Starbucks, signed on with activist groups led by Common Sense Media, the National Association for the Advancement of Colored People, and the Anti-Defamation League to remove their ads from Facebook for the month of July. This was an attempt, the groups said, the put pressure on Facebook to change its policies about hate speech and misinformation.

However, Facebook has proven resilient against so many big advertisers leaving its platform. Although MarketWatch reported that its stock tanked briefly in June when the boycott was announced, total ad revenue has remained basically steady throughout the boycott, according to Forbes. The social media giant is set to publish its second-quarter earnings report on Thursday, which should show whether the boycott had any kind of major effect on Facebook’s bottom line.

As such, Axios reported, Common Sense Media thinks this suggests a problem with competition in the advertising sphere and has “highlighted how much control Facebook has over online advertising.”

The group is urging committee members to raise the boycott with the Facebook executive when he comes to testify on Wednesday.