The opinion explains things pretty well for the most part. The elements of a negligence claim are (1) duty, (2) breach, (3) causation and (4) damages. The first element was the issue here.
Whether a duty exists is a question of law for the judge to decide. Generally, we're all obliged to act as a reasonably careful person would act under the same or similar circumstances. That's the standard common law duty of ordinary care. However, there's an assload of exceptions generally known as no-duty rules.
One exception provides that, absent a "special relationship," there's no duty to affirmatively render aid to another. If you're walking along a beach and see someone drowning in the lake, you're free to point and laugh as far as the common law is concerned. A related but distinct no-duty rule provides that there's no obligation to control another person's conduct or to protect another from a third-person's criminal acts. That's the no-duty rule that ultimately killed the plaintiffs' claims in this case.
Yes, courts definitely consider feasibility and cost in deciding whether to impose a duty in a particular case. The judge in this case did that when he ruled that exposing MySpace to negligence liability in cases like this "would of course stop MySpace's business in its tracks and close this avenue of communication . . . ."
Personally, I'm all the way down with the judge's reasoning. Shutting down MySpace means no more comedic gold such as
this, and that would be completely unacceptable.