The discovery tool StumbleUpon has just passed the 1 billion stumbles per month milestone. A month ago, the site was seeing some 800 million stumbles per month, according to The Next Web.

The huge increase in traffic can be at least partially attributed to the fact that StumbleUpon now has iPhone, iPad, and Android apps for mobile stumbling. Mobile StumbleUpon is growing 40 percent every month, according to the company.

StumbleUpon expanded its discovery tool to Android in November 2010, allowing users to stumble upon new apps to download form the Android Marketplace. At the time, there was an iOS version in the works, but we haven't heard much about it since.

The initiative was a very interesting one and is worth noting again as the site passes such a huge milestone. Although StumbleUpon will always have its root in site discovery, its algorithm can definitely be put to use in a variety of scenarios.

StumbleUpon was founded in November 2001 by Garrett Camp, Geoff Smith, Justin LaFrance, and Eric Boyd in Calgary, Canada. It launched in 2002 and didn't move to the San Francisco Bay Area until 2006. It was acquired by eBay a year later for $75 million, only to be bought back by Camp, Smith, and several investors in 2009.