To use Facebook and Twitter for emergency purposes, you have to set it up as such. You have to set it up to get updates from various emergency management authorities and from the people you intend to "survive with." You can get quick updates from your people to know how/what they're doing, where they are, etc., and to give the same information about your location, etc. to a large number of people quickly and efficiently.

Some people use Facebook to network for work, or to meet guys/gals, etc. If that's what you normally do on these sites, you shouldn't expect much during emergencies. Similarly, you don't have control over what the people you'd like to survive with use Facebook for. They might use Facebook primarily to share knock knock jokes. But you'll be able to keep a line of communication open, and it's an efficient way to see who's OK and who's not.

DB