Invoking my Mod Superpowers: the culprit is
where the post is currently number 3 on the list. It accounted for a whopping 25% of the total traffic to Math.SE in the last 24 hours, referring more users to this site than did Google in the same time period.
(Somewhat scary considering that, if I read it correctly, the link was only posted there some 5 hours ago.)
Facebook and Twitter together accounted for less than a quarter of the above.
What I am surprised about is that the person who posted it to YCombinator linked the Long URL, and not the short permalink with his UID encoded in it. If he had only shared the link properly, he would've earned a Publicist badge by now. (But of course, had he done that, we would've been able to track down what his username is on Math.SE and be able to blame this all on him...)
One final summary of the traffic pattern about this:
Using March and August of this year as the baseline, we see that Google directed the vast majority of traffic our way (over 50%). (MathOverflow and StackOverflow are roughly tied for a distant second at about 3% each).
Between July 29 and August 7, the average daily page view is about 135% of the baseline. Between July 29 and August 4, the figure is about 155% of the baseline. During 7/29 - 8/7, non-search-engine traffic dominated the directed traffic, though Google still is the single site contributing the most traffic (the search term "batman equation" is the single most popular search term, 5 times the number of the second most popular). Between 7/29 and 8/4, Ycombinator and Facebook together accounted for as much traffic as Google's ~20% of the share.
To answer Asaf's question in the comments, between July 29 and August 4, the one single question accounted for about 30% of our total page views.