Facebook has enabled their safety check feature:
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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.
A college classmate on 9/11 died in the Windows of the World restaurant. Her death may be a statistic to you, but it is not to me.
I remember trying to call friends in NYC that day, all the exchanges were overloaded. I'm grateful that we now have ways to connect without worrying about possibly preventing emergency calls from going through.