There can also be an emergency backup generator, with pull cord, for Twitter. To be green, the generator's fuel can be animal poop (preferably from birds). Tweet!

An alert system's usefulness is proportional to its reach. For an online widely distributed alert system, Twitter's one of the few services that would fit that goal.

"distributed" in what sense? Technology-wise, Twitter is highly centralized - and a rather unreliable center at that.

A Twitter-like UI that would simply send email to all your emergency contacts might be a lot more reliable and much easier to scale.

I'd say, why not have the choice in addition to existing services?