In Silicon Valley, according to Siva Vaidhyanathan, a UVA professor whose book about Facebook, “Antisocial Media,” is due out in September, “Tools are technologies that generate other technologies.” When I asked an engineer friend who builds “developer tools” for his definition, he noted that a tool is distinct from a product, since a product is “experienced rather than used.” The iTunes Store, he said, is a product: “there are lots of songs you can download, but it’s just a static list.” A Web browser, by contrast, is a tool, because “the last mile of its use is underspecified.”