Clearly a family emergency plan is important. We have discussed that a) no news is good news, b) not to make assumptions about who has fetched the kids from school daycare, c) meeting spots on our street, neighborhood, village, and region.  
 
Make sure your family plan starts with the most basic things. 1) How to evacuate the house in case of fire, and for kids 2) being able to recall important details - name, parents name, address, phone numbers.

XKCD recently made a comic about earthquakes and Twitter. 
http://xkcd.com/723/ 
I think it illustrates the effect of social media on natural disasters well. 
 
Nice #I'mOK idea. You make a good point that not everyone may look to Twitter for updates. I had relatives in NYC on 9/11, and email was the primary method of family status communication at the time, but it was not an immediate thing at all. 
 
During the hurricane Katrina aftermath, even when the cell phone voice systems were down, text messages still went through "sometimes". Any disaster that affects the communications infrastructure will affect our ways of communicating about it.