Downton Abbey's 2-hour premiere last night got 7.9 million viewers. Viewership-wise that beat everything else in primetime with the exception of CBS in its timeslot. That's 96% more viewers than the season 2 premiere and 46% above the season 2 Christmas special, the previous series high.
I expected it to increase from last season since the hype has been stupid-high for it this season, but that's still a pretty incredible number (Even if it doesn't mean much to PBS). Could you imagine how big it would be if it aired the same day as the UK?