I can see a problem with this, when I open the link the URL remains the same (i.e. the "referrer" part is not updated). If I were to share it I'd just copy/paste the URL, in effect meaning that I won't appear as a "node" in the graph. I wonder if that's why the graph seems very "centralized" around a few points.
Unless it's because I refused to share my ___location. They should explain why they need it beforehand, I would never allow that by default (I only understood the point once I saw the map).
EDIT: Also, it will probably not take the retweets and similar into account, as the URL will remain the same. Overall it's an interesting concept but I doubt it'll provide any worthwhile data.
This would have been an easy fix too. They should give everyone who loads the page a new referrer ID and associated it with that user's IP. Then just redirect or use ___pushState to modify the URI in the browser.
It's a shame that they made such an obvious mistake as it's a very cool visualization otherwise.
My guess is that they use the referrer only as a point of origin on the map. Each subsequent user to hit that referrer id plots on the map in sequence from the original, based on their geolocation. They probably do not plot users who do not share their locale.
Nothing wrong with being alarmed by a site that automatically plays music and wants to know where you live.
If the cashier at Subway or any other stranger with whom you had minor contact one day asked where you lived wouldn't you be more than just a little alarmed?
What if a major retailer asked for your ZIP code at the register? This happens to me often and is much more analogous to the situation than a cashier asking where I live.
Unless it's because I refused to share my ___location. They should explain why they need it beforehand, I would never allow that by default (I only understood the point once I saw the map).
EDIT: Also, it will probably not take the retweets and similar into account, as the URL will remain the same. Overall it's an interesting concept but I doubt it'll provide any worthwhile data.