It is important not to think of any one social media tool or device as essential, because your ability to use any one of them in a given disaster may depend on fast-changing variables such as bandwidth availability, electricity supplies, and damage to cell towers and other infrastructure. Rather, think of them as a range of options from which you can mix and match.
Even the most basic phones now usually include cameras, which staffers on the scene can use to document damage to facilities, fallen trees or other obstructions, hazards such as fires, wind, etc. Remember that in any kind of disaster it is better whenever possible to use text messages (which use almost no bandwidth) than regular voice calls, in order to reduce bandwidth congestion. So encourage staffers to experiment with texting now.
These phones, which are becoming more and more the norm, add computing power, functions such as video, and GPS chips, which can be critical because of establishing precisely where the user is. One of their advantages is that they can store disaster information that users can still access even if the local communications infrastructure is totally destroyed.
It's a good idea to make your disaster plan available in a phone-friendly format that staffers can download in advance (making certain they always update as the plan evolves!) so that they will literally have it in hand when disaster strikes. (Of course, keeping an updated, printed copy in various locations is also advised.)
In an absolute worst case, if power and Internet connections are lost, you can still have some rudimentary information-sharing if your staffers all have cheap walkie-talkies.
One model for such a system, the District of Columbia Emergency Radio Network (DCERN)  , was pioneered by D.C. literary agent Bill Adler. The members all go outside one Sunday night a month, turn on their walkie-talkies, tune to the same channel, and practice relaying messages. They have been able to blanket the District of Columbia, and can pass the information worldwide if needed through a link to ham radio operators.
Twitter has become a widely-used disaster information-sharing tool because of its ability to concisely convey location-based, real-time information. However, its usability was initially limited by the lack of uniform standards to describe and identify the types of information being relayed, making it very hard to aggregate and share information, particularly by automatic computer-based distribution.
During the Haiti earthquake, the Universities of Colorado and California's Empower the Public With Information in a Crisis (EPIC) program rushed into use the "Tweak the Tweet" (TtT) taxonomy they had been developing. TtT introduced uniform "hashtags" such as #loc (location) #need (type of aid needed) into emergency Tweets. The addition of this standard syntax for information makes it machine-readable, which also means that the data can be automatically disseminated, a critical factor in disasters. TtT's significance extends beyond Twitter use in disasters, creating a model for systematizing the content of other social media alerts. Because of the system's simplicity and logic, it is likely the public will rapidly adopt TtT use in a disaster (to our knowledge, no one to date has launched a formal education campaign about use of Twitter hashtags of any sort, but new ones spread quickly and are used continually to classify and search Twitter contact, simply because users quickly grasp their utility and the logic of how to create short, accurate ones). However, it is also logical that a formal outreach program by public radio stations might speed their dissemination and adoption. Using TtT to ask listeners to contribute information about a given geographic location in a disaster would likely result in quick adoption of the tags by users.
You may want to consider two types of strategy regarding Twitter use.
Many organizations have adopted Yammer, a no-cost/low-cost variation on Twitter developed specifically for use behind firewalls for secure communication.  Preferably, you would use Yammer internally, and, as many public stations already have done, also create a Twitter account for communication with listeners both in normal times and disasters.
Note that for your Twitter feed to have credibility with listeners, it must observe the spirit of social media and both "follow" other users (especially listeners) as well as interact with them on a regular basis, creating a dialogue rather than simply using the Twitter feed as alternative way of broadcasting your program schedule.
Facebook is a critical social media tool in disasters, in part because its user base dwarfs those for other social apps and in part because its API allows so many types of content (Twitter feeds, YouTube Videos, etc.), to come together on a Facebook page. The fact that Facebook comments are not limited to 140 characters also allows for more detailed information sharing.
Facebook's "Discussion" feature allows listeners to discuss a situation or event and offer their own observations. When Facebook added the "Places" feature to its mobile edition, it became possible for those posting Status Updates to give their location, which could be critical in a disaster for "hyperlocal information."
As community station KUYI Hopi Radio (see sidebar) found, one of Facebook's most important uses in a disaster can be its ability to alert family and friends outside of your listening area to what the current situation is. That kind of global outreach, by the way, is one of the greatest substantive contributions a station can provide to the overall response effort: the better job it does using social media and other tools that are hosted outside the local area (and therefore less affected by bandwidth or power limits) to keep people elsewhere informed, the less need they will feel to make phone calls to affected friends and family. That preserves as much bandwidth as possible for first responders.
Perhaps worse than having no disaster plan is having an obsolete one. Using a wiki as the platform for your station's disaster plan will increase the likelihood that it remains current, because any and all users can contribute content based on findings from response to new disasters as they occur.
During an actual disaster, if you have adequate bandwidth and enough staffers have computer access, the wiki can also be used as the means for staffers to share information and quickly and easily access all information from past plans and disasters. 
CrisisWiki has become a central global repository for disaster response which you can use both as a source of information and as a model for your own wiki.
A wiki is another example of a social media tool that the station could use as a critical component of its day-to-day management and then switch seamlessly to disaster use. Its advantages include radically reducing email and increased collaboration. 
Crowdmap is another example of a disaster tool that you can use as part of your regular newsgathering and to test the waters in terms of crowdsourcing information, specifically because it also was created as a tool for visualizing election data. It is a free application created by the Africa-based Ushahidi Project, specifically as a way to organize crowd-sourced information into interactive maps and timelines.
Crowdmap has been used frequently in disaster management, most notably during the Haiti earthquake. It is particularly helpful for the two aspects of disaster coverage most applicable to involving the public in information-gathering: managing crowd-sourced input and visualizing hyperlocal information. Best of all, it is hosted by Ushahidi, so that it can still function and be accessible even if your local bandwidth is limited or infrastructure crippled.
If, for example, you use Crowdmap as a way of organizing listeners' information around election coverage, you can both enrich your coverage and gain valuable experience in using the tool that could be applied during a disaster.
Since they were first used widely during the London subway bombing, photos (and now video) from cell phones can provide invaluable situational awareness, although their use will be severely limited when there are bandwidth constraints. Photos can be gathered on services such as Twitpic if a tag associating them with a disaster are added ( as with these from the 2010 Boulder wildfires, #boulderfire ). Videos could be even more helpful because of the ability to capture motion and pan, but you need to discuss in advance of a disaster what kind of visual information would be helpful. Such a discussion would be an excellent topic for a "Meet-Up" (see section 6.1) to lay the groundwork for an involving the public in your coverage.
Finally, if your resources allow, consider building a smartphone app similar to the ones created by many public radio stations, including WBUR in Boston: it combines buttons for a wide range of functions which have made it a must for listeners (live stream, news, program listings and podcast, making a contribution, and... "assignments"). Clicking "assignments" lets listeners learn about crowdsourced content the station would like them to contribute. This kind of all-purpose app takes the guesswork out of connecting with the station – especially important in high-stress situations such as disasters – and increases the bond between listener and station.
If you're interested in a similar app, it was created by the Public Radio Exchange (PRX) which is making it available to other stations either by building one for your station, or by inviting use of the open source code to build a similar app in-house or hire your own developer to work with the code.