@can @chrisgeidner X's angle is anti-trust. The advertisers are working together as part of a group (the Global Alliance for Responsible Media) that sets standards for social media advertising. If the group decides that X isn't brand-safe, none of the members can advertise on it. Are they illegally working together to restrain trade?

I'm not a lawyer, but I wouldn't think it's a strong case. Usually when companies conspire together, they're working to increase their profits - for example, "none of us will sell cereal for less than $10/box so consumers have to pay more". X is alleging that they're working together in a way that decreases their profits by forcing them to pay higher advertising rates on other social media sites.

X is arguing that they must individually negotiate brand safety standards with social media companies, not combine their efforts - but that means those companies all paying to duplicate their efforts, which presumably X hopes they won't do.