Social discovery service StumbleUpon has hit a new milestone with the news that it now has more than 20 million users. The service’s user base has doubled in size since May 2010, with more than 1,000 stumbles per second occurring at peak times of the day.

StumbleUpon now serves as many stumbles in one hour as it did in its first year. Since last year, the company has increased its employee base from 40 to more than 80, while the number of stumbles per month has more than doubled from 400 million to more than 1 billion.

The company visualized some key points in its history in a new chart (which you can click to enlarge), including the point where it hit 1 million members (July 2006), its acquisition by eBay (May 2007) and when it hit 10 billion total stumbles (July 2009).

StumbleUpon, which raised $17 million in funding in March, updated its iPhone and iPad apps earlier this month to include the Explore Box, a feature that was launched in August.