In Silicon Valley, “disruption” is giving way to “building.” What will be built? : In 2009, Marc Andreessen—a Silicon Valley entrepreneur and angel investor best known for cofounding Netscape, in 1994, at the age of twenty-two—announced that he would be starting a venture-capital firm. “I’m crossing over into the dark side,” he said, jokingly, to the PBS talk-show host Charlie Rose.