Koum was in his office, on the second floor of his house, in Santa Clara, in Silicon Valley, when he saw that it was working. “It registered itself, connected, and messages started flowing between two people,” he recalled. “I was, like, Holy shit, I just built a messenger for iPhone.” WhatsApp became WhatsApp. “Almost everybody went from, like, Oh, this is useless to Oh, this is very useful,” Koum said. The network came alive.