The best feature about Facebook & Twitter is that you only have to send (and be charged for) one text message which will then be recieved by everyone who is "following" you. This is much more convenient then calling all your "out-or-area" friends and family to let them know what's going on.

I have my "Merriwether" persona Facebook and Twitter accounts linked so that anything I post on my facebook account automatically gets sent out on my Twitter feed, which makes it even more convenient.

Many emergency services providers are also setting up feeds to get out information to the people in their areas. They post stuff about possible dangers (weather, flooded road, WILDFIRES, etc) and how to escape, where to go for help and stuff like that.

Here in Houston for the Blast-clan it's useful. If you don't have many friends or family out of state or in other distant locations or if the fire/police/other emergency units don't have feeds then maybe these social networks aren't all that useful for you. Security-wise, you can set them up with no personal information at all and just allow known friends/family to access it.

-Blast