mildly disregarding the specific situation;
situations like this really beg the question of where a person, and our government for that matter, draws the line between forms of communication. because a threat like this made on airport grounds would not beg the same public reaction. i would even go as far as to say that a telephone call to a local politician and making a death threat would also beg a more strong response with less people defending the instigator.
but when it boils down to it, no matter what form of communication you use, a threat is a thread and should be treated as one. joking is something you do with friends or at worst some peers that sort of know you. a joke is not something to be pulled on an international company that has increased focus on security by 100% since september 11th. it's just not a justifiable thing to do. whether you think it's a joke, or she was too young to understand. if shes too young then her parents shouldn't have permitted her using social media. at the end of the day, she got very lucky more wasnt done to her.