The question isn't unambigiously answered, but there's a top of good stuff here (first link in the article):

http://www.extremetech.com/computing/123929-just-how-big-are...

"In short, porn sites cope with astronomical amounts of data. The only sites that really come close in term of raw bandwidth are YouTube or Hulu, but even then YouPorn is something like six times larger than Hulu."