The Netflix experience is probably different than Youtube. I suspect the average viewer per Netflix account is significantly higher than one. An account is, after all, $8 a month. You can only rate once per Netflix account. An other defence of House of Cards is that many people will wait until they have finished the 13 episode season before rating the show. Only the insane have already viewed the 11 hours of House of Cards made available by Netflix 5 days ago. Still, you can rate the series, or even an episode, without having viewed the entire series. I suspect it is more than 0.6% that have done so.
So with that disclaimer out of the way, here is my conclusion: Best case scenario, 20 million people have viewed 1 or more episodes of the 2013 US version of House of Cards.
Math: 120,000 X 168 = 20,160,000 people. Since the average rating for House of Cards is 4.6 stars out 5, we can assume most of those viewers will eventually view each of the 13 episode.
These days, you usually need a karaoke machine to reach 20 million people an episode.