Fewer advertisers seem to be appearing on Breitbart, which may be a result of some ad networks and brand safety companies blocking it in recent months, citing hate speech violations or deeming the content too inflammatory.

Breitbart had about 1,300 advertisers on its website last month, showing about 2,600 display ads, according to data from Moat Pro, a digital ad intelligence product. That was down from 3,300 advertisers and 11,500 display ads in November. It is not clear if the boycott has had a financial impact, although the top thousand domestic advertisers accounted for 73 percent of display advertising spending last year, Kantar Media said.

Brands are finding that the mechanics of online ads, typically placed through a complex system of agencies and third-party networks that resemble a stock exchange, can be difficult to explain to consumers.

Nordstrom, for example, responded on Twitter to complaints about its ads on Breitbart by saying, “Although we no longer advertise with Breitbart, some of our ads may appear due to the nature of how online ads work.” Nordstrom said by email that while it had blacklisted Breitbart through the vendors it uses to buy and serve ads, it bought some ads through online exchanges that allow sites to conceal their addresses, which may account for the appearances.

“At this time, Nordstrom doesn’t block those types of advertising opportunities because many prominent, well-respected and legitimate websites choose to use these tactics,” Emily Sterken, a spokeswoman, said by email. “Our teams are looking into how we might address this situation.”

Ben Winkler, the chief investment officer at the agency OMD, said other brands that showed up on Breitbart in spite of blacklists probably had several parties handling their digital ad spending, including local marketing affiliates, publishers and public relations agencies. For a major company, “there are probably, at any given time, 100 different campaigns being run by a dozen different departments across a thousand websites and a dozen agencies,” he said. “To think that every single one of those players is religiously and slavishly using the approved site list or is blacklisting Breitbart is unlikely.”