I saw this happen with the London bombings. A colleague was almost in tears because her son lived in London and she couldn't contact him on the phone. It turned out he was fine, as approximately everybody else in the city was too.
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.