StumbleUpon is a discovery engine that uses collaborative filtering to recommend web content to users. It started in 2001 with $1.2 million in funding and has since received $17 million more in 2011. The company was acquired by eBay in 2007 but was later bought back by its founders. StumbleUpon has grown to over 20 million users and drives a large percentage of media traffic, surpassing Facebook in some measures. The company employs recommendation algorithms and social sharing to help users discover new websites and videos that similar users enjoy.