A Twitter account is putting the pressure on brands advertising on Breitbart. It has already gotten responses from Modcloth, SoFi and Nest, which say they are pulling their advertising. Others aren’t so sure: Nissan said it doesn’t plan to change its marketing mix. Brands and the agencies that work for them are caught in a tough place when it comes to ads on websites like Breitbart, which regularly publish sexist, racist and anti-Semitic posts. But because of programmatic and the tangled web it weaves, many may not even know what they’re advertising next to.