As B.O. Mojo relates:
... With close to $209 million through its eighth week, Dragon has grossed nearly five times what it did on its opening weekend, which is the biggest multiplier of any 2010 nationwide release thus far and, among major animated releases, the biggest multiplier since Finding Nemo. Its average weekend drop-off rate has been a slim 26 percent. ...
Dragon is on pace to replace Kung Fu Panda as DreamWorks Animation's highest-grossing non-Shrek release. Panda generated $215.4 million, though Dragon's unlikely to top Panda's attendance due to its inflated 3D prices. Overall, 3D has accounted for 67 percent of Dragon's total gross. Sift the 3D ticket price premium out and Dragon's $209 million would adjust to the equivalent of less than $170 million.
Word of mouth kept Dragon soaring, but the ad campaign launching the feature didn't quite do the job during its first weekend. I donno, maybe it was the Vikings that were a hard sell. It's been a long time since burly men in spiked helmets commanded big bucks at the box office.