You might be surprised; I know Alexa isn't exactly an infallible oracle, but they list groupon.com as averaging between the 200th and 400th most visited site globally, and 53rd in the US. "ranking.com", whoever they are, says between 150 and 75. I'd feel confident saying they're at least in the top 500, and that requires a fair bit of horsepower. Now, they could probably solve a lot of their problems with smarter engineering before building a data center, but they certainly have traffic.