If you were head of a psychiatric ward and had an employee you knew was telling patients "Boy, I sure wish someone would kill as many black people as they could", you would absolutely share responsibility when on of them did exactly that.

If you were deliberately pairing that employee with patients who had shown violent behaviour on the basis of "they both seem to like violence", you would absolutely share responsibility for that violence.

This isn't a matter of "there's just so much content, however can we check it all?".

Reddit has hosted multiple extremist and dangerous communities, claiming "we're just the platform!" while handing over the very predictable post histories of mass shooters week after week.

YouTube has built an algorithm and monetisation system that is deliberately designed to lure people down rabbit holes then done nothing to stop it luring people towards domestic terrorism.

It's a lawsuit against companies worth billions. They're not being executed. There are grounds to accuse them of knowingly profiting from the grooming of terrorists and if they want to prove that's not the case, they can do it in court.