Earlier quoted context omitted.
Some people have Wifi but not cell phone signal. A single tweet has very robust, existing infrastructure, over the “one email per person in a mailing list” style of email; and SMS is DoA when there’s no cell phone.
No phone, no TV, only a PC that they sit at logged in to Twitter all day refreshing their feed in case a local emergency comes up?
Compare that to email, which you’d get hundreds of messages a day and only care about 3 of them, so you’re not constantly checking.
Edit: dude you live in Australia. Do you know how people in the Bush get their emergency updates that they need to respond to urgently? I bet at least some of them use Twitter or something similar to it. They're not huddled around their weather radio all day (and that doesn't cover everything). Because I had family in rural California and that is exactly how they got theirs. Twitter and Facebook.