Hacker News Reader

Interesting. I show 10.5M-ish monthly uniques to SurveyMonkey, as of March 2011: http://siteanalytics.compete.com/surveymonkey.com/

Where did you get the 20M uniques from?

Keep in mind that Compete is almost always laughably wrong.

It's usually under-estimates more than it over-estimates, at least for US traffic.