It's terribly inaccurate, based on my experience. For the Feb 2011 (most recent available stat they have), they were wrong by -314%. They should make the fact that their service is inaccurate obvious; otherwise, they're misleading many people.

Their data points are from 2 million people. In the U.S. alone, there are 230 million+ interet users: http://www.google.com/publicdata?ds=wb-wdi&met=it_net_us...

I only took basic stats courses in college, but I know enough to know that's nowhere close to a big enough sample size to be statistically significant.