Now it's more relevant to say that Xbox One is "still the fastest selling next gen console", since it sold more in the US than PS4 did in December.
December 2013 NPD highlights include:
• Total combined retail spend on the Xbox platform (Xbox One and Xbox 360) in December reached $1.39 billion, 50 percent of the combined software, hardware and accessories spend in the U.S. (Source: NPD Group, December 2013).
• During the month of December, Xbox One held six of the top 10 spots on the new generation console game title list: “Battlefield 4,” “Forza Motorsport 5,” “Dead Rising 3,” “Ryse: Son of Rome,” “Madden NFL 25” and “Call of Duty: Ghosts” (Source: NPD Group, December 2013).
This is going to be a good race, ladies and gentlemen... still plenty of hard work to do for the rest of the world (and to maintain our momentum in the US), but no doubt - we're the fastest selling next gen console in the US.