From Wikipedia:
Hybrid Theory was released in the United States on October 24, 2000 following the debut of "One Step Closer" on the radio.[22] It entered the U.S. Billboard 200 charts at #16 in late 2000,[23] and was certified gold by the RIAA five weeks after its release.[4] In 2001, Hybrid Theory sold 4.8 million copies in the United States, making it the best-selling album of the year,[24][25] and it was estimated that the album continued selling 100,000 copies per week in early 2002.[3] Throughout the following years, the album continued to sell at a fast pace and was certified diamond by the RIAA in 2005 for selling 10 million copies in the U.S.[26] It has also sold 29 million copies worldwide as of 2009, which made it the band's best-selling album and the best-selling debut album of the 21st century.
Pretty damn impressive. 29 million copies worldwide.