I think the distinction here is between people and businesses. Is it the fault of people on social media for the acts of others? No. Is it the fault of social media for cultivating an environment that radicalizes people into committing mass shootings? Yes. The blame here is on the social medias for not doing more to stop the spread of this kind of content. Because yes even though that won't stop this kind of content from existing making it harder to access and find will at least reduce the number of people who will go down this path.