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.
The world is full of real people, with real understandable concerns for their families and friends.
Not only that, but it isn't about the number who were killed anyway. It is about the probability of your friend being in that location at the time of the disaster which is much, much higher.
No one thinks 'I wonder if Jack was killed, he was at that park today'. They think 'I wonder if Jack was in the CBD, he works nearby!'
Statistically validated concerns or not, the very real effect of this part of human nature is that the phone infrastructure goes down and that sucks for everyone.