To expand on 0x4A4D's comment, traffic spikes from reddit and other external links are a fact of life on Mathematica.SE. The canonical example was Amatya's xkcd question, with 143,000 page views and approaching 400 upvotes just for the question. But it has been a bit of a tradition here to have bust-out graphics questions, as well as previously when some of us were in the Mathematica tag on Stack Overflow - check out the famous/infamous Where's Waldo? question.
You can get the list of these famous questions literally by viewing the list of questions that got the "Famous Question" badge. In addition, browsing the list of questions that got someone the Publicist badge will show you most of the questions that went viral because someone posted the link somewhere, as well as who posted the link.
Moderators have access to pageview and other traffic stats but we aren't permitted to share specifics. What I can tell you is that the public traffic numbers show that Mathematica.SE averages about 5,000 visits/day on a typical weekday, and the numbers for Sunday and Monday will average a bit more than five times that by the time Monday is over in UTC timezone. The first day of the xkcd question frenzy was a bit more than double that, so in the ballpark of 60,000 visits in a day compared with about 5,000 normally.
The data that moderators have access to is not much different from the one publicly available at quαntcast. If you're reasonably familiar with the site's history — or at least, the ones that went viral — you should be able to figure out the peaks yourself.
