Twitter was used during the Fargo floods with its own hashtag. For small emergencies or ones that only take up a city or state this will work well. For national emergencies a single hashtag may be overloaded – think 9/11 or pandemic. Also a nationa blackout caused by sun spots or grid failure may bring down twitter or peoples access.
Red Cross has a web site to register that you are safe.
Hand held personal radios are nice for families but since they each use different channels it may be hard to reach a neighbor – for get a state trooper.
CBs are not as good as they used to be. Most states no longer monitor channel #9 like they used to. Post event they may be useful.
HAM radios are great and they have been life savers.
INMARSAT and other SATCOM units work but are expen$ive. I do not think they are ready for a mass event where hundreds or thousands of folks would use them for extended periods of time.
FAX machines. Yes fax. Send a fax to four people each who make sure the other 3 got it via phone and then fax the info on to four other people in a tree network. Good for getting out info.
eMail lists. Some even send out quarterly emails as tests. This is great when web sited die or things happen. Not everyone is on twitter.
Low power FM. These are small stations with 5 mile radius for serving a samll area.
Its not much but it is a start. Stop by the pandemic group or even fluwiki if you like.