Silicon Valley venture capital firm Andreessen Horowitz raised $650 million in just three weeks for its second fund, according to a report carried by Reuters this morning. The fund is more than twice the size of the firm’s original fund and comes just 16 months after Marc Andreessen and Ben Horowitz started the Menlo Park, CA-based firm in July, 2009. Andreessen, who co-authored the Mosaic Web browser and co-founded Netscape Communications, and Horowitz, who worked at Netscape and went on to co-found Loudcloud/Opsware with Andreessen (later sold to HP), have invested in Web startups and enterprise software startups. Their investments include Zynga, Foursquare and Skype.