In 2015, Jason Citron, a computer programmer, was struggling to break through in the video game industry. The new multiplayer game he had created with his development studio, Hammer & Chisel, was not catching on. So Mr. Citron engineered an abrupt about-face. He laid off his company’s game developers, turned the game’s chatting feature into its sole product and gave it a mysterious name — Discord. “I think at the time we had maybe six users,” Mr. Citron said in an interview. “It wasn’t clear that it was going to work.” At first, Discord was popular only with other gamers. But more than six years later, driven in part by the pandemic, it has exploded into the mainstream. While adults working from home flocked to Zoom, their children were downloading Discord to socialize with other young people through text and audio and video calls in groups known as servers. The platform has more than 150 million active users each month — up from 56 million in 2019 — with nearly 80 percent logging in from outside North America. It has expanded from gamers to music aficionados, students and cryptocurrency enthusiasts. In September, Discord, which is based in San Francisco, said it was raising $500 million in funding, valuing the company at $14.7 billion, according to PitchBook, a market data provider. It more than doubled its work force in 2021, to about 650 people.