[Amorymeltzer]

A longer, more involved article from late September has a bit more information: http://highscalability.com/blog/2015/9/28/how-facebook-tells...

>If a friend is in the same area then a push notification is sent asking if they are OK.

One thing that I'd like to understand is how "area" 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's way. An earthquake is almost easier - there'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'd love to see that.

[chipperyman573]

They probably are very generous - I'd imagine the entire country was defined as the "area". The system doesn't work very well if there's false negatives and people don't see it, and if someone sees it but shouldn't all they have to do is hit that they're OK.

[xxbondsxx]

Not quite @chipperyman573. A few points to make here:

-- Since we only have city-level location for most users, declaring the area isn't as hard as drawing on a map. We usually just select a number of cities, regions, states, or countries that are affected by the crisis.

-- We always allow people to declare themselves into the crisis (or out) in case our geolocation prediction is inaccurate. This means we can be a bit more selective with the geographic area, since we want to be pretty high signal with our notifications. We actually use notification click-through and conversion rates as downstream signals on how well a launch went.

-- For something like Paris, we actually just selected the whole city and launched. Especially with the media reporting "Paris terror attacks," this seemed like a good fit.

[c22]

I got a safety check because my home city is listed as "Paris", but I was not actually in Paris. I consistently reject requests to share my location though, so it's possible Facebook has more precise information about other users.

[smackfu]

France is a pretty big country. If you are in Nice, you are a nine hour drive from Paris. It would seem a bit silly to ask if you were okay, and not someone in Belgium who is much, much closer.

[cortesoft]

It is only about 248 thousand square miles... which is smaller than Texas.