If 1,000,000 people try to figure out whether the 1 person they know in a city of 2,000,000 are ok within a very short time-frame then the phone network will stop functioning.
A place to post a simple 'I'm ok' will cut down tremendously on the number of calls that will then still need to be made. Unfortunately (1) not everybody has facebook and (2) a 'no show' will now certainly lead to a phonecall when before it would be assumed the person was ok so I'm not sure whether it helps or not but the basic idea is probably a good one. It would be even better if this functionality was available outside of having a facebook account.