Texting uses smaller packets of data, and often works when voice does not. It's been suggested that you use texting when cell lines are overloaded after an emergency (i.e. 9/11) for this very reason, as the texts are more likely to reach the intended recipient. Also, text messages are stored on the recipient's phone, whereas most voicemails have to be accessed by calling into a voicemail box, which would be hard to do if 20 million people are trying to call their kids/parents etc. after a disaster for example.

So, for all we know the kids tried to call, but were unable to do so and used their noggins to text their facebook pages... regardless, it worked and that's all that matters.