10% of American Netflix subscribers have seen House of Cards

"Netflix has been mum on viewership of House of Cards, preferring so far not to break out the number of subscribers who have watched it or how many hours they’ve watched. The company has said, however, that the program is its most-watched show. And research from financial services firm Cowen and Company suggests that Netflix’s original programming is already starting to pay dividends for the streaming service.
Cowen surveyed 1,200 people last week, of which about 28 percent were paying Netflix subscribers, and another 18 percent had access to a Netflix streaming account. Of those, about 10 percent had watched House of Cards during the first few weeks it was available. And they’re not just watching one episode — on average, Netflix viewers had streamed an average of six episodes of the new original series.
Viewers generally rated the series well, with 80 percent saying it was “good” or “exceptional.” Viewers also liked Netflix making all 13 episodes of the first season available at the same time. According to the survey, 90 percent viewed that release plan positively, as they gobbled up multiple episodes per viewing session."

Source:  Techcrunch, 19th February 2013

Note - Since he only asked approx 360 subscribers I consider this to be a small(ish) sample