A longer, more involved article from late September has a bit more information: <a href="http:&#x2F;&#x2F;highscalability.com&#x2F;blog&#x2F;2015&#x2F;9&#x2F;28&#x2F;how-facebook-tells-your-friends-youre-safe-in-a-disaster-in.html" rel="nofollow">http:&#x2F;&#x2F;highscalability.com&#x2F;blog&#x2F;2015&#x2F;9&#x2F;28&#x2F;how-facebook-tells...</a><p>&gt;If a friend is in the same area then a push notification is sent asking if they are OK.<p>One thing that I&#x27;d like to understand is how &quot;area&quot; is defined.  The limiting factor is, of course, finding out something has happened, but someone at Facebook has to draw a border that encompasses everyone at risk but minimizes those out of harm&#x27;s way.  An earthquake is almost easier - there&#x27;s (relatively) rapid data on the epicenter and size - but in an event and place like Paris, there have got to be some hard calls made in real time.  Who are the people and teams involved there, and how do they make those decisions?  I&#x27;d love to see that.