The American Red Cross has released results of a survey about the use of social media during disasters or emergencies. The PDF report of its findings can be found here:
Some interesting figures are contained within the report, including that over 75% of respondents would expect help to arrive within an hour if they requested help via a social media network. Personally, I think this expectation is overly optimistic, but it would hinge upon who was in a user's respective social graph.
When Hurricane Ike hit The Woodlands 2 years ago, I realized that we'd likely lose power for a while and find it difficult to communicate with the outside world. Prior to the storm's landfall in Texas, I e-mailed all of my friends & family, and directed them towards my Twitter page to stay up to date with our progress during & after storm.
This turned out to be a prudent decision, because we did in fact lose power & most cellular connectivity for several days. Thankfully, SMS messages are surprisingly resilient following natural disasters (they require much less cellular infrastructure to receive & transmit relative to voice & data services). This allowed me to update Twitter throughout the storm & its aftermath simply by texting the service.
For those interested in seeing my account of "events on the ground", I captured all my tweets from the storm in a blog post last September to mark the 1 year anniversary of Hurricane Ike:
Since that time, I've followed my friends' descriptions of earthquakes on the west coast and crippling blizzards on the east coast, all from my Twitter & Facebook activity streams. When you want to tell a large group of people you're ok after a disaster, it's tough to find a more efficient way than with one of these social networks.